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Debt (Tables) | 12 Months Ended
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Dec. 31, 2023
Debt Instrument [Line Items] |
Schedule of Other Short-term Borrowings | Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Amount outstanding | $475 |  | $642 |  | $— |  | $— |  | $318 |  | $290
Weighted average interest rates | 5.5% |  | 4.6% |  | N/A |  | N/A |  | 5.4% |  | 4.5%
Available credit facility capacity | $525 |  | $358 |  | $150 |  | $100 |  | $82 |  | $110
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Maximum amount outstanding (based on daily outstanding balances) | $793 |  | $665 |  | $70 |  | $— |  | $349 |  | $325
Average amount outstanding (based on daily outstanding balances) | $386 |  | $411 |  | $2 |  | $— |  | $157 |  | $153
Weighted average interest rates | 5.2% |  | 2.1% |  | 5.3% |  | —% |  | 5.1% |  | 1.6%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2023 |  | 2022
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.25%, due 2024 | $500 |  |  | $500 |  |  | $— |  |  | $500 |  |  | $500 |  |  | $— |
3.4%, due 2025 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
5.5%, due 2025 | 50 |  |  | 50 |  |  | — |  |  | 50 |  |  | 50 |  |  | — |
4.1%, due 2028 | 500 |  |  | 500 |  |  | — |  |  | 500 |  |  | 500 |  |  | — |
3.6%, due 2029 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
2.3%, due 2030 | 400 |  |  | 400 |  |  | — |  |  | 400 |  |  | 400 |  |  | — |
5.7%, due 2033 (b) | 300 |  |  | 300 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100 |  |  | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |
6.3%, due 2034 | 125 |  |  | 125 |  |  | — |  |  | 125 |  |  | 125 |  |  | — |
6.25%, due 2039 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
4.7%, due 2043 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
3.7%, due 2046 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.5%, due 2049 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.1%, due 2051 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
| 3,975 |  |  | 3,975 |  |  | — |  |  | 3,675 |  |  | 3,675 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.05%, due 2027 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3%, due 2029 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
1.95%, due 2031 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3.95%, due 2032 | 600 |  |  | — |  |  | 600 |  |  | 600 |  |  | — |  |  | 600 |
4.95% due 2033 (c) | 300 |  |  | — |  |  | 300 |  |  | — |  |  | — |  |  | — |
6.25%, due 2034 | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |  |  | 100 |
6.375%, due 2037 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
7.6%, due 2038 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
4.1%, due 2044 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
3.65%, due 2050 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
| 3,100 |  |  | — |  |  | 3,100 |  |  | 2,800 |  |  | — |  |  | 2,800 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d) | 300 |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a) | 200 |  |  | — |  |  | — |  |  | 200 |  |  | — |  |  | — |
Alliant Energy 3.875% convertible senior notes, due 2026 (e) | 575 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) | 300 |  |  | — |  |  | — |  |  | 300 |  |  | — |  |  | — |
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f) | 300 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a) | 350 |  |  | — |  |  | — |  |  | 350 |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 9 |  |  | — |  |  | — |  |  | 17 |  |  | — |  |  | — |
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023) | — |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
Other, 1% at December 31, 2023, due 2024 to 2025 | — |  |  | — |  |  | — |  |  | 1 |  |  | — |  |  | — |
| 2,034 |  |  | — |  |  | — |  |  | 1,668 |  |  | — |  |  | — |
Subtotal | 9,109 |  |  | 3,975 |  |  | 3,100 |  |  | 8,143 |  |  | 3,675 |  |  | 2,800 |
Current maturities | (809) |  |  | (500) |  |  | — |  |  | (408) |  |  | — |  |  | — |
Unamortized debt issuance costs | (54) |  |  | (21) |  |  | (19) |  |  | (45) |  |  | (21) |  |  | (19) |
Unamortized debt (discount) and premium, net | (21) |  |  | (9) |  |  | (11) |  |  | (22) |  |  | (8) |  |  | (11) |
Long-term debt, net (g) | $8,225 |  |  | $3,445 |  |  | $3,070 |  |  | $7,668 |  |  | $3,646 |  |  | $2,770 |
(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments. (c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs. (d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap. (e)Refer to “Convertible Senior Notes” below for additional information. (f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (g)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt | At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):
| 2024 |  | 2025 |  | 2026 |  | 2027 |  | 2028
IPL | $500 |  | $300 |  | $— |  | $— |  | $500
WPL | — |  | — |  | — |  | 300 |  | —
AEF | 309 |  | — |  | 200 |  | — |  | 300
Alliant Energy parent company | — |  | — |  | 575 |  | — |  | —
Alliant Energy | $809 |  | $300 |  | $775 |  | $300 |  | $800
IPL [Member] |
Debt Instrument [Line Items] |
Schedule of Other Short-term Borrowings | Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Amount outstanding | $475 |  | $642 |  | $— |  | $— |  | $318 |  | $290
Weighted average interest rates | 5.5% |  | 4.6% |  | N/A |  | N/A |  | 5.4% |  | 4.5%
Available credit facility capacity | $525 |  | $358 |  | $150 |  | $100 |  | $82 |  | $110
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Maximum amount outstanding (based on daily outstanding balances) | $793 |  | $665 |  | $70 |  | $— |  | $349 |  | $325
Average amount outstanding (based on daily outstanding balances) | $386 |  | $411 |  | $2 |  | $— |  | $157 |  | $153
Weighted average interest rates | 5.2% |  | 2.1% |  | 5.3% |  | —% |  | 5.1% |  | 1.6%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2023 |  | 2022
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.25%, due 2024 | $500 |  |  | $500 |  |  | $— |  |  | $500 |  |  | $500 |  |  | $— |
3.4%, due 2025 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
5.5%, due 2025 | 50 |  |  | 50 |  |  | — |  |  | 50 |  |  | 50 |  |  | — |
4.1%, due 2028 | 500 |  |  | 500 |  |  | — |  |  | 500 |  |  | 500 |  |  | — |
3.6%, due 2029 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
2.3%, due 2030 | 400 |  |  | 400 |  |  | — |  |  | 400 |  |  | 400 |  |  | — |
5.7%, due 2033 (b) | 300 |  |  | 300 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100 |  |  | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |
6.3%, due 2034 | 125 |  |  | 125 |  |  | — |  |  | 125 |  |  | 125 |  |  | — |
6.25%, due 2039 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
4.7%, due 2043 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
3.7%, due 2046 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.5%, due 2049 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.1%, due 2051 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
| 3,975 |  |  | 3,975 |  |  | — |  |  | 3,675 |  |  | 3,675 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.05%, due 2027 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3%, due 2029 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
1.95%, due 2031 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3.95%, due 2032 | 600 |  |  | — |  |  | 600 |  |  | 600 |  |  | — |  |  | 600 |
4.95% due 2033 (c) | 300 |  |  | — |  |  | 300 |  |  | — |  |  | — |  |  | — |
6.25%, due 2034 | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |  |  | 100 |
6.375%, due 2037 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
7.6%, due 2038 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
4.1%, due 2044 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
3.65%, due 2050 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
| 3,100 |  |  | — |  |  | 3,100 |  |  | 2,800 |  |  | — |  |  | 2,800 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d) | 300 |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a) | 200 |  |  | — |  |  | — |  |  | 200 |  |  | — |  |  | — |
Alliant Energy 3.875% convertible senior notes, due 2026 (e) | 575 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) | 300 |  |  | — |  |  | — |  |  | 300 |  |  | — |  |  | — |
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f) | 300 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a) | 350 |  |  | — |  |  | — |  |  | 350 |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 9 |  |  | — |  |  | — |  |  | 17 |  |  | — |  |  | — |
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023) | — |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
Other, 1% at December 31, 2023, due 2024 to 2025 | — |  |  | — |  |  | — |  |  | 1 |  |  | — |  |  | — |
| 2,034 |  |  | — |  |  | — |  |  | 1,668 |  |  | — |  |  | — |
Subtotal | 9,109 |  |  | 3,975 |  |  | 3,100 |  |  | 8,143 |  |  | 3,675 |  |  | 2,800 |
Current maturities | (809) |  |  | (500) |  |  | — |  |  | (408) |  |  | — |  |  | — |
Unamortized debt issuance costs | (54) |  |  | (21) |  |  | (19) |  |  | (45) |  |  | (21) |  |  | (19) |
Unamortized debt (discount) and premium, net | (21) |  |  | (9) |  |  | (11) |  |  | (22) |  |  | (8) |  |  | (11) |
Long-term debt, net (g) | $8,225 |  |  | $3,445 |  |  | $3,070 |  |  | $7,668 |  |  | $3,646 |  |  | $2,770 |
(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments. (c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs. (d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap. (e)Refer to “Convertible Senior Notes” below for additional information. (f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (g)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt | At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):
| 2024 |  | 2025 |  | 2026 |  | 2027 |  | 2028
IPL | $500 |  | $300 |  | $— |  | $— |  | $500
WPL | — |  | — |  | — |  | 300 |  | —
AEF | 309 |  | — |  | 200 |  | — |  | 300
Alliant Energy parent company | — |  | — |  | 575 |  | — |  | —
Alliant Energy | $809 |  | $300 |  | $775 |  | $300 |  | $800
WPL [Member] |
Debt Instrument [Line Items] |
Schedule of Other Short-term Borrowings | Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper classified as short-term debt was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Amount outstanding | $475 |  | $642 |  | $— |  | $— |  | $318 |  | $290
Weighted average interest rates | 5.5% |  | 4.6% |  | N/A |  | N/A |  | 5.4% |  | 4.5%
Available credit facility capacity | $525 |  | $358 |  | $150 |  | $100 |  | $82 |  | $110
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2023 |  | 2022 |  | 2023 |  | 2022 |  | 2023 |  | 2022
Maximum amount outstanding (based on daily outstanding balances) | $793 |  | $665 |  | $70 |  | $— |  | $349 |  | $325
Average amount outstanding (based on daily outstanding balances) | $386 |  | $411 |  | $2 |  | $— |  | $157 |  | $153
Weighted average interest rates | 5.2% |  | 2.1% |  | 5.3% |  | —% |  | 5.1% |  | 1.6%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2023 |  | 2022
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.25%, due 2024 | $500 |  |  | $500 |  |  | $— |  |  | $500 |  |  | $500 |  |  | $— |
3.4%, due 2025 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
5.5%, due 2025 | 50 |  |  | 50 |  |  | — |  |  | 50 |  |  | 50 |  |  | — |
4.1%, due 2028 | 500 |  |  | 500 |  |  | — |  |  | 500 |  |  | 500 |  |  | — |
3.6%, due 2029 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
2.3%, due 2030 | 400 |  |  | 400 |  |  | — |  |  | 400 |  |  | 400 |  |  | — |
5.7%, due 2033 (b) | 300 |  |  | 300 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100 |  |  | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |
6.3%, due 2034 | 125 |  |  | 125 |  |  | — |  |  | 125 |  |  | 125 |  |  | — |
6.25%, due 2039 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
4.7%, due 2043 | 250 |  |  | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |
3.7%, due 2046 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.5%, due 2049 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
3.1%, due 2051 | 300 |  |  | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |
| 3,975 |  |  | 3,975 |  |  | — |  |  | 3,675 |  |  | 3,675 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.05%, due 2027 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3%, due 2029 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
1.95%, due 2031 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
3.95%, due 2032 | 600 |  |  | — |  |  | 600 |  |  | 600 |  |  | — |  |  | 600 |
4.95% due 2033 (c) | 300 |  |  | — |  |  | 300 |  |  | — |  |  | — |  |  | — |
6.25%, due 2034 | 100 |  |  | — |  |  | 100 |  |  | 100 |  |  | — |  |  | 100 |
6.375%, due 2037 | 300 |  |  | — |  |  | 300 |  |  | 300 |  |  | — |  |  | 300 |
7.6%, due 2038 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
4.1%, due 2044 | 250 |  |  | — |  |  | 250 |  |  | 250 |  |  | — |  |  | 250 |
3.65%, due 2050 | 350 |  |  | — |  |  | 350 |  |  | 350 |  |  | — |  |  | 350 |
| 3,100 |  |  | — |  |  | 3,100 |  |  | 2,800 |  |  | — |  |  | 2,800 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through March 2024, 6% at December 31, 2023 (with Alliant Energy as guarantor) (d) | 300 |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a) | 200 |  |  | — |  |  | — |  |  | 200 |  |  | — |  |  | — |
Alliant Energy 3.875% convertible senior notes, due 2026 (e) | 575 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a) | 300 |  |  | — |  |  | — |  |  | 300 |  |  | — |  |  | — |
AEF 5.95% senior notes, due 2029 (with Alliant Energy as guarantor) (a)(f) | 300 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 3.6% senior notes, due 2032 (with Alliant Energy as guarantor) (a) | 350 |  |  | — |  |  | — |  |  | 350 |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 9 |  |  | — |  |  | — |  |  | 17 |  |  | — |  |  | — |
AEF 3.75% senior notes (with Alliant Energy as guarantor) (Retired in 2023) | — |  |  | — |  |  | — |  |  | 400 |  |  | — |  |  | — |
Other, 1% at December 31, 2023, due 2024 to 2025 | — |  |  | — |  |  | — |  |  | 1 |  |  | — |  |  | — |
| 2,034 |  |  | — |  |  | — |  |  | 1,668 |  |  | — |  |  | — |
Subtotal | 9,109 |  |  | 3,975 |  |  | 3,100 |  |  | 8,143 |  |  | 3,675 |  |  | 2,800 |
Current maturities | (809) |  |  | (500) |  |  | — |  |  | (408) |  |  | — |  |  | — |
Unamortized debt issuance costs | (54) |  |  | (21) |  |  | (19) |  |  | (45) |  |  | (21) |  |  | (19) |
Unamortized debt (discount) and premium, net | (21) |  |  | (9) |  |  | (11) |  |  | (22) |  |  | (8) |  |  | (11) |
Long-term debt, net (g) | $8,225 |  |  | $3,445 |  |  | $3,070 |  |  | $7,668 |  |  | $3,646 |  |  | $2,770 |
(a)Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption. (b)In September 2023, IPL issued $300 million of 5.7% senior debentures due 2033. The net proceeds from the issuance were used to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt, for general corporate purposes and/or were placed in money market fund investments. (c)In March 2023, WPL issued $300 million of 4.95% debentures due 2033. The debentures were issued as green bonds, and an amount equal to or in excess of the net proceeds was disbursed for the development and acquisition of its solar EGUs. (d)In January 2023, AEF entered into a $300 million interest rate swap maturing in January 2026 to mitigate interest rate risk. Under the terms of the swap, AEF exchanged a variable interest rate for a fixed interest rate of 3.93% on a portion of its variable-rate term loan borrowings. In December 2023, AEF retired the remaining $100 million variable-rate term loan borrowings. Refer to Note 15 for additional information on the interest rate swap. (e)Refer to “Convertible Senior Notes” below for additional information. (f)In November 2023, AEF issued $300 million of 5.95% senior notes due 2029. The net proceeds from AEF’s issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes. (g)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt | At December 31, 2023, long-term debt maturities for 2024 through 2028 were as follows (in millions):
| 2024 |  | 2025 |  | 2026 |  | 2027 |  | 2028
IPL | $500 |  | $300 |  | $— |  | $— |  | $500
WPL | — |  | — |  | — |  | 300 |  | —
AEF | 309 |  | — |  | 200 |  | — |  | 300
Alliant Energy parent company | — |  | — |  | 575 |  | — |  | —
Alliant Energy | $809 |  | $300 |  | $775 |  | $300 |  | $800

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