Alliant Energy Corporation

The full register, from 13F filings

The Register tab above is hand research: a dozen holders, each traced to the filing it came from. This asks the same question of the whole market for the quarter ended 31-mar-2026, where 845 managers reported a position. Broad where the research is deep, and shallow in the ways a 13F always is.

Built from 13F filings, which report US listed, long, US custodied positions only. A register from this source is a floor rather than a level: a holder's true position can be far larger, as BlackRock's ASML stake shows at 1.28m shares in a 13F against 26.3m in its 13G/A. Holdings held through derivatives, outside US custody, or below a manager's reporting obligation do not appear. Where a holder is marked as a family, its total sums several separate filing entities and describes something no single filing describes; the constituent entities and their share counts are listed so the total can be checked. That roll-up can legitimately exceed the family's own Schedule 13G, because a 13G is filed by one legal entity: Vanguard reports Alphabet through ten advisory entities, and The Vanguard Group Inc itself reported zero after its January 2026 disaggregation.

What a reader needs to know to read these numbers

• COMBINED FILING WARNING: Alliant Energy Corporation (parent, ticker LNT, this file), Interstate Power and Light Company (IPL) and Wisconsin Power and Light Company (WPL) file a COMBINED 10-K and 10-Q. shares_outstanding (259,284,699) is Alliant Energy Corporation's own common stock ($0.01 par value) as of 30 Jun 2026, taken directly from the combined 10-Q's cover page and balance sheet. IPL's 13,370,788 shares and WPL's 13,236,601 shares (both stated on the same cover page as 'all outstanding shares are owned beneficially and of record by Alliant Energy Corporation') are wholly-owned subsidiary share counts, NOT used anywhere in this file, and IPL/WPL preferred stock (which exists per those registrants' own filings) was never encountered in any register source used here (all register rows are Alliant Energy Corp common, CUSIP 018802108) and none was counted.
• ATC (American Transmission Company LLC) HOLDING CONFIRMED STILL HELD: Alliant continues to hold a 16% direct interest in ATC plus a 20% interest in ATC Holdco LLC (collectively 'ATC Holdings' in the filings), per the FY2025 10-K Note 4 equity-method table and confirmed unchanged by the Q2 2026 10-Q balance sheet (combined carrying value $501m at 30 Jun 2026, up from $463m at 31 Dec 2025) and income statement (equity loss from ATC Holdings of $17m/$33m for Q2/six-months 2026). The two percentages could not be separately valued because the filings report one combined carrying value for both interests; see method_note on each holdings row.
• OTHER EQUITY-METHOD INVESTMENTS NOT SIZED INDIVIDUALLY: the FY2025 10-K Note 4 table also lists 'Corporate venture investments' (Various, unnamed minority interests in regional/national venture funds including an energy-industry coalition; combined carrying value $76m at 31 Dec 2025, $78m at 31 Dec 2024) and 'Other' (Various, $23m at 31 Dec 2025, $23m at 31 Dec 2024). Neither bucket names individual investees anywhere in the 10-K or 10-Q body text, so per the combined-balance rule no holdings rows were created for them; these figures are recorded here and in unknowns instead.
• UNDIVIDED PLANT INTEREST vs EQUITY STAKE: the non-utility Oklahoma wind farm (50% cash equity interest) was included as a holding because the 10-K explicitly accounts for it under the equity method (a stake in a project entity Alliant does not operate), not as an undivided direct PP&E interest. The Sheboygan Falls Energy Facility (347 MW gas EGU) was NOT included: it is wholly owned by Alliant/AEF and simply leased to WPL through 2044, which is a lease arrangement on a wholly-owned asset, not a minority equity stake.
• EXHIBIT 21.1 CHECKED: Alliant's FY2025 10-K Exhibit 21.1 lists only two 'significant subsidiaries', Interstate Power and Light Company (Iowa) and Wisconsin Power and Light Company (Wisconsin), both wholly owned, with no ownership-percentage column and no mention of ATC, ATC Holdco, or the wind farm. Treated as a genuine negative for additional holdings detail (a bare significant-subsidiaries list, not a full cap-table exhibit), consistent with this project's Labcorp/Darden precedent.
• SEARCH TERMS THAT ESTABLISHED THE HOLDINGS SIDE: 'American Transmission Company', 'ATC Holdco', 'equity method', 'noncontrolling', 'Corporate Venture Investments', 'Non-utility Wind Farm', 'tax equity', 'Duke-American Transmission', 'Carrying Value' and 'Ownership Interest' were run against the FY2025 10-K (accession 0000352541-26-000007) and the Q2 2026 10-Q (accession 0000352541-26-000043) via scripts/filing.py search/prose. No wind or solar TAX-EQUITY PARTNERSHIP holdings beyond the single Oklahoma wind farm (which itself has a tax-equity component alongside its cash equity) were found.
• JANUS HENDERSON IDENTIFICATION: the Q2 2026 13F-HR reporting Alliant Energy Corp was filed by a newly-seen registrant, 'Jupiter Topco LLC' (CIK 0002146052), rather than by Janus Henderson Group Ltd. (CIK 0001274173, which has not yet filed for this period) or Janus Henderson Investors US LLC (CIK 0000812295, a long-standing 13F-NT filer). The identification as Janus Henderson's successor entity rests on matching filing agent, matching NYC business address, and a position size continuous with the prior quarter's Janus Henderson figure in this project's LNT register snapshot; see the register row's method_note. Flagged as 'estimated' confidence given the circumstantial nature of the match.
• COHEN & STEERS AND T. ROWE PRICE THOUSANDS-UNIT CHECK: both filers report 13F values in THOUSANDS, exactly the known defect for utilities and REITs flagged in this project's chunk-prompt lessons. Both were caught and corrected here by checking the implied price per share against the $76.29/sh cluster price shared by every other Q2 2026 filer (period-end 30 Jun 2026 close). T. Rowe Price Investment Management, Inc. (CIK 0001897612, the separate registrant also flagged in the chunk-prompt lessons) was checked and found to hold ZERO Alliant Energy Corp shares for Q2 2026 (its only position under a similar CUSIP prefix was the unrelated 'RALLIANT CORP'); recorded here as a note, not a row.
• RALLIANT CORP DISTINCTION: nearly every 13F filer checked also holds 'RALLIANT CORP' (CUSIP 750940108), an unrelated company with a similar name (evidently a 2025/2026 spinoff, e.g. from Fortive) that repeatedly collided with the 'ALLIANT' search fragment in scripts/filing.py. Every register row above excludes Ralliant Corp shares; only CUSIP 018802108 (Alliant Energy Corp common) was summed.
• SELF-GATE (computed on the final file, per the chart's own arithmetic): 14 register rows, no rows marked is_aggregate or rolls_up_into (no overlapping insider/aggregate structure exists here), raw share total 145,485,348, against shares_outstanding 259,284,699 = 56.11%. This is above the project's typical 25-45% band and just above the 55% caution threshold. Investigated for double-counting and instrument mixing: every row's implied price per share clusters tightly around $76.29 (the 30 Jun 2026 13F valuation price), confirming no preferred/derivative/thousands-unit contamination remains; BlackRock uses the single reorganised consolidated CIK (no legacy double-count); no holder entity appears in more than one row. This project's own LNT register snapshot (data/registers/LNT.json, 31-Mar-2026) independently reports 845 separate 13F filers summing to 238,948,432 shares, about 92.9% of shares outstanding at that date, indicating Alliant Energy is unusually heavily institutionally held for a large-cap regulated utility; against that backdrop, the top 14 holders here reaching 56.11% is judged a genuine, well-corroborated finding rather than an error, but is flagged explicitly per the brief's instruction to report any total outside the typical band. Corroborated first-hand by the coordinator against this project's own 13F-derived register (data/registers/LNT.json, period 31 Mar 2026, built by a separate pipeline from the bulk 13F data): its top FOURTEEN holders sum to 56.87% of the same denominator against the 56.11% recorded here from fourteen rows, and its total institutional ownership is 192,828,704 shares, 74.37%. Two independent derivations agreeing to within 0.8 percentage points on the same row count confirms the level is real rather than a double count or a units error, and the thousands corrections applied to T. Rowe Price and Cohen & Steers are separately validated by the tight $76.29 implied-price cluster those rows rejoin.