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Debt
 9 Months Ended
Sep. 30, 2025
Debt [Line Items]  
Debt DEBT
NOTE 7(a) Short-term Debt - In March 2025, Alliant Energy, IPL and WPL reallocated credit facility capacity amounts to $550 million for Alliant Energy at the parent company level, $350 million for IPL and $400 million for WPL, within the $1.3 billion total commitment. Information regarding commercial paper and borrowings under the single credit facility classified as short-term debt was as follows (dollars in millions):
September 30, 2025Alliant EnergyIPLWPL
Amount outstanding$192$—$192
Weighted average interest rates4.3%N/A4.3%
Available credit facility capacity$1,108$350$208
Alliant EnergyIPLWPL
Three Months Ended September 30202520242025202420252024
Maximum amount outstanding (based on daily outstanding balances)$372$330$78$—$297$—
Average amount outstanding (based on daily outstanding balances)$263$197$9$—$177$—
Weighted average interest rates4.5%5.4%4.6%N/A4.5%N/A
Nine Months Ended September 30
Maximum amount outstanding (based on daily outstanding balances)$741$632$141$19$297$390
Average amount outstanding (based on daily outstanding balances)$417$320$32$1$187$87
Weighted average interest rates4.6%5.5%4.6%5.5%4.5%5.5%

NOTE 7(b) Long-term Debt - In March 2025, AEF entered into a $300 million variable rate (5% as of September 30, 2025) term loan credit agreement (with Alliant Energy as guarantor), which expires in March 2026. This term loan credit agreement amended and restated the term loan credit agreement that expired in March 2025, and retired the $300 million variable rate term loan set forth therein, which was classified as a non-cash financing activity. AEF’s restated term loan credit agreement includes an option to increase the amount outstanding with one or more additional term loans in an aggregate amount not to exceed $100 million.

In May 2025, IPL issued $600 million of 5.6% senior debentures due 2035. The net proceeds from this issuance were used for the retirement of IPL’s senior debentures in July 2025 ($50 million, 5.5%) and August 2025 ($250 million, 3.4%), to reduce cash amounts received from its sale of accounts receivable program and commercial paper classified as long-term debt, and for general corporate purposes.

In September 2025, IPL issued $300 million of 5.6% senior debentures due 2055. The net proceeds from this issuance were used to reduce cash amounts received from its sale of accounts receivable program, reduce its outstanding commercial paper and for general corporate purposes.

In September 2025, Alliant Energy issued $725 million of junior subordinated notes due 2056. The notes will bear interest at 5.75% until April 1, 2031. The interest rate will reset every 5 years beginning April 1, 2031, to equal the then-current five-year U.S. Treasury rate plus a spread of 2.077%, provided the interest rate will not reset below 5.75%. A portion of the net proceeds from this issuance were placed in time deposits and money market fund investments pending the retirement of AEF’s $200 million of 1.40% senior notes, AEF’s $300 million variable rate term loan and Alliant Energy’s $575 million of 3.875% convertible senior notes, each of which matures in March 2026 and is expected to be retired at or prior to maturity. The remainder of the net proceeds were used to reduce outstanding commercial paper and for general corporate purposes.

Convertible Senior Notes - In May 2025, Alliant Energy issued $575 million of 3.25% convertible senior notes (the 2028 Notes), which are senior unsecured obligations, and used the net proceeds from the issuance to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. The 2028 Notes will mature on May 30, 2028 unless earlier converted or repurchased, and no sinking fund is provided for the 2028 Notes. Alliant Energy may not redeem the 2028 Notes prior to the maturity date. Holders may convert their 2028 Notes at their option at any time prior to the close of business on the business day immediately preceding March 1, 2028 only under the following circumstances:

•during any calendar quarter commencing after the calendar quarter ending on September 30, 2025 (and only during such calendar quarter), if the last reported sale price of Alliant Energy’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day during such period;
•during the 5 business day period after any 10 consecutive trading day period (the “measurement period”) in which the trading price (as defined in the related Indenture) per $1,000 principal amount of 2028 Notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of Alliant Energy’s common stock and the conversion rate on each such trading day; or
•upon the occurrence of specified corporate events.

On or after March 1, 2028 until the close of business on the business day immediately preceding the maturity date, holders may convert all or any portion of their 2028 Notes at any time, regardless of the foregoing circumstances. Upon conversion of the 2028 Notes, Alliant Energy will pay cash up to the aggregate principal amount of the 2028 Notes to be converted and pay or deliver, as the case may be, cash, shares of its common stock or a combination of cash and shares of its common stock, at its election, in respect of the remainder, if any, of its conversion obligation in excess of the aggregate principal amount of the 2028 Notes being converted.
The initial conversion rate is 13.1773 shares of common stock per $1,000 principal amount of 2028 Notes (equivalent to an initial conversion price of approximately $75.89 per share of Alliant Energy’s common stock). The conversion rate is subject to adjustment in some events but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, Alliant Energy will, in certain circumstances, increase the conversion rate for a holder who elects to convert its 2028 Notes in connection with such a corporate event.

If Alliant Energy undergoes a fundamental change (as defined in the related Indenture), then, subject to certain conditions, holders of the 2028 Notes may require Alliant Energy to repurchase for cash all or any portion of its 2028 Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 2028 Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date.

As of September 30, 2025, the conditions allowing holders of the 2028 Notes and Alliant Energy’s convertible senior notes due 2026 (the 2026 Notes) to convert their respective notes were not met, and the 2028 Notes were classified as “Long-term debt, net” and the 2026 Notes were classified as “Current maturities of long-term debt,” on Alliant Energy’s balance sheet. As of September 30, 2025, the net carrying amount was $569 million and $574 million, with unamortized debt issuance costs of $6 million and $1 million, and the estimated fair value (Level 2) was $593 million and $619 million for the 2028 Notes and 2026 Notes, respectively. As of September 30, 2025, no shares of Alliant Energy’s common stock and 27,905 shares of Alliant Energy’s common stock related to the potential conversion of the 2028 Notes and 2026 Notes, respectively, were included in diluted EPS based on Alliant Energy’s average stock prices and the relevant terms of the respective notes.
IPL [Member]  
Debt [Line Items]  
Debt DEBT
NOTE 7(a) Short-term Debt - In March 2025, Alliant Energy, IPL and WPL reallocated credit facility capacity amounts to $550 million for Alliant Energy at the parent company level, $350 million for IPL and $400 million for WPL, within the $1.3 billion total commitment. Information regarding commercial paper and borrowings under the single credit facility classified as short-term debt was as follows (dollars in millions):
September 30, 2025Alliant EnergyIPLWPL
Amount outstanding$192$—$192
Weighted average interest rates4.3%N/A4.3%
Available credit facility capacity$1,108$350$208
Alliant EnergyIPLWPL
Three Months Ended September 30202520242025202420252024
Maximum amount outstanding (based on daily outstanding balances)$372$330$78$—$297$—
Average amount outstanding (based on daily outstanding balances)$263$197$9$—$177$—
Weighted average interest rates4.5%5.4%4.6%N/A4.5%N/A
Nine Months Ended September 30
Maximum amount outstanding (based on daily outstanding balances)$741$632$141$19$297$390
Average amount outstanding (based on daily outstanding balances)$417$320$32$1$187$87
Weighted average interest rates4.6%5.5%4.6%5.5%4.5%5.5%

NOTE 7(b) Long-term Debt - In March 2025, AEF entered into a $300 million variable rate (5% as of September 30, 2025) term loan credit agreement (with Alliant Energy as guarantor), which expires in March 2026. This term loan credit agreement amended and restated the term loan credit agreement that expired in March 2025, and retired the $300 million variable rate term loan set forth therein, which was classified as a non-cash financing activity. AEF’s restated term loan credit agreement includes an option to increase the amount outstanding with one or more additional term loans in an aggregate amount not to exceed $100 million.

In May 2025, IPL issued $600 million of 5.6% senior debentures due 2035. The net proceeds from this issuance were used for the retirement of IPL’s senior debentures in July 2025 ($50 million, 5.5%) and August 2025 ($250 million, 3.4%), to reduce cash amounts received from its sale of accounts receivable program and commercial paper classified as long-term debt, and for general corporate purposes.

In September 2025, IPL issued $300 million of 5.6% senior debentures due 2055. The net proceeds from this issuance were used to reduce cash amounts received from its sale of accounts receivable program, reduce its outstanding commercial paper and for general corporate purposes.
WPL [Member]  
Debt [Line Items]  
Debt DEBT
NOTE 7(a) Short-term Debt - In March 2025, Alliant Energy, IPL and WPL reallocated credit facility capacity amounts to $550 million for Alliant Energy at the parent company level, $350 million for IPL and $400 million for WPL, within the $1.3 billion total commitment. Information regarding commercial paper and borrowings under the single credit facility classified as short-term debt was as follows (dollars in millions):
September 30, 2025Alliant EnergyIPLWPL
Amount outstanding$192$—$192
Weighted average interest rates4.3%N/A4.3%
Available credit facility capacity$1,108$350$208
Alliant EnergyIPLWPL
Three Months Ended September 30202520242025202420252024
Maximum amount outstanding (based on daily outstanding balances)$372$330$78$—$297$—
Average amount outstanding (based on daily outstanding balances)$263$197$9$—$177$—
Weighted average interest rates4.5%5.4%4.6%N/A4.5%N/A
Nine Months Ended September 30
Maximum amount outstanding (based on daily outstanding balances)$741$632$141$19$297$390
Average amount outstanding (based on daily outstanding balances)$417$320$32$1$187$87
Weighted average interest rates4.6%5.5%4.6%5.5%4.5%5.5%