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Debt (Tables)
 12 Months Ended
Dec. 31, 2020
Debt Instrument [Line Items]  
Schedule of Other Short-term Borrowings Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions):
Alliant EnergyIPLWPL
December 31202020192020201920202019
Amount outstanding$389$337$—$—$257$168
Weighted average interest rates0.2%1.9%N/AN/A0.1%1.8%
Available credit facility capacity$611$663$250$250$43$132
Alliant EnergyIPLWPL
For the year ended202020192020201920202019
Maximum amount outstanding (based on daily outstanding balances)$495$601$8$50$266$195
Average amount outstanding (based on daily outstanding balances)$292$453$—$—$117$93
Weighted average interest rates0.7%2.5%0.5%2.8%0.7%2.4%
Schedule of Long-term Debt Long-term debt, net as of December 31 was as follows (dollars in millions):
20202019
Alliant EnergyIPLWPLAlliant EnergyIPLWPL
Senior Debentures (a):
3.25%, due 2024$500 $500 $— $500 $500 $— 
3.4%, due 2025250 250 — 250 250 — 
5.5%, due 202550 50 — 50 50 — 
4.1%, due 2028500 500 — 500 500 — 
3.6%, due 2029 300 300 — 300 300 — 
2.3%, due 2030 (b)400 400 — — — — 
6.45%, due 2033100 100 — 100 100 — 
6.3%, due 2034125 125 — 125 125 — 
6.25%, due 2039300 300 — 300 300 — 
4.7%, due 2043250 250 — 250 250 — 
3.7%, due 2046 300 300 — 300 300 — 
3.5%, due 2049 300 300 — 300 300 — 
3.65% (Retired in 2020)— — — 200 200 — 
3,375 3,375 — 3,175 3,175 — 
Debentures (a):
2.25%, due 2022250 — 250 250 — 250 
3.05%, due 2027300 — 300 300 — 300 
3%, due 2029 350 — 350 350 — 350 
6.25%, due 2034100 — 100 100 — 100 
6.375%, due 2037300 — 300 300 — 300 
7.6%, due 2038250 — 250 250 — 250 
4.1%, due 2044250 — 250 250 — 250 
3.65%, due 2050 (c)350 — 350 — — — 
4.6% (Retired in 2020)— — — 150 — 150 
2,150 — 2,150 1,950 — 1,950 
Other:
AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d)300 — — — — — 
Corporate Services 3.45% senior notes, due 2022 (a)75 — — 75 — — 
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)400 — — 400 — — 
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e)200 — — — — — 
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)300 — — 300 — — 
Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)31 — — 38 — — 
AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020)— — — 300 — — 
Other, 1% at December 31, 2020, due 2021 to 20252 — — 2 — — 
1,308 — — 1,115 — — 
Subtotal6,833 3,375 2,150 6,240 3,175 1,950 
Current maturities(8)— — (657)(200)(150)
Unamortized debt issuance costs(41)(22)(14)(37)(21)(11)
Unamortized debt (discount) and premium, net(15)(8)(6)(13)(7)(6)
Long-term debt, net (f)$6,769 $3,345 $2,130 $5,533 $2,947 $1,783 

(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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes.
(c)In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes.
(d)In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020.
(e)In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(f)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions):
20212022202320242025
IPL$—$—$—$500$300
WPL—250———
Corporate Services—75———
AEF83084089—
Alliant Energy$8$633$408$509$300
IPL [Member]  
Debt Instrument [Line Items]  
Schedule of Other Short-term Borrowings Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions):
Alliant EnergyIPLWPL
December 31202020192020201920202019
Amount outstanding$389$337$—$—$257$168
Weighted average interest rates0.2%1.9%N/AN/A0.1%1.8%
Available credit facility capacity$611$663$250$250$43$132
Alliant EnergyIPLWPL
For the year ended202020192020201920202019
Maximum amount outstanding (based on daily outstanding balances)$495$601$8$50$266$195
Average amount outstanding (based on daily outstanding balances)$292$453$—$—$117$93
Weighted average interest rates0.7%2.5%0.5%2.8%0.7%2.4%
Schedule of Long-term Debt Long-term debt, net as of December 31 was as follows (dollars in millions):
20202019
Alliant EnergyIPLWPLAlliant EnergyIPLWPL
Senior Debentures (a):
3.25%, due 2024$500 $500 $— $500 $500 $— 
3.4%, due 2025250 250 — 250 250 — 
5.5%, due 202550 50 — 50 50 — 
4.1%, due 2028500 500 — 500 500 — 
3.6%, due 2029 300 300 — 300 300 — 
2.3%, due 2030 (b)400 400 — — — — 
6.45%, due 2033100 100 — 100 100 — 
6.3%, due 2034125 125 — 125 125 — 
6.25%, due 2039300 300 — 300 300 — 
4.7%, due 2043250 250 — 250 250 — 
3.7%, due 2046 300 300 — 300 300 — 
3.5%, due 2049 300 300 — 300 300 — 
3.65% (Retired in 2020)— — — 200 200 — 
3,375 3,375 — 3,175 3,175 — 
Debentures (a):
2.25%, due 2022250 — 250 250 — 250 
3.05%, due 2027300 — 300 300 — 300 
3%, due 2029 350 — 350 350 — 350 
6.25%, due 2034100 — 100 100 — 100 
6.375%, due 2037300 — 300 300 — 300 
7.6%, due 2038250 — 250 250 — 250 
4.1%, due 2044250 — 250 250 — 250 
3.65%, due 2050 (c)350 — 350 — — — 
4.6% (Retired in 2020)— — — 150 — 150 
2,150 — 2,150 1,950 — 1,950 
Other:
AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d)300 — — — — — 
Corporate Services 3.45% senior notes, due 2022 (a)75 — — 75 — — 
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)400 — — 400 — — 
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e)200 — — — — — 
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)300 — — 300 — — 
Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)31 — — 38 — — 
AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020)— — — 300 — — 
Other, 1% at December 31, 2020, due 2021 to 20252 — — 2 — — 
1,308 — — 1,115 — — 
Subtotal6,833 3,375 2,150 6,240 3,175 1,950 
Current maturities(8)— — (657)(200)(150)
Unamortized debt issuance costs(41)(22)(14)(37)(21)(11)
Unamortized debt (discount) and premium, net(15)(8)(6)(13)(7)(6)
Long-term debt, net (f)$6,769 $3,345 $2,130 $5,533 $2,947 $1,783 

(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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes.
(c)In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes.
(d)In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020.
(e)In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(f)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions):
20212022202320242025
IPL$—$—$—$500$300
WPL—250———
Corporate Services—75———
AEF83084089—
Alliant Energy$8$633$408$509$300
WPL [Member]  
Debt Instrument [Line Items]  
Schedule of Other Short-term Borrowings Information regarding Alliant Energy’s, IPL’s and WPL’s commercial paper, and Alliant Energy’s and WPL’s borrowings under the single credit facility, classified as short-term debt was as follows (dollars in millions):
Alliant EnergyIPLWPL
December 31202020192020201920202019
Amount outstanding$389$337$—$—$257$168
Weighted average interest rates0.2%1.9%N/AN/A0.1%1.8%
Available credit facility capacity$611$663$250$250$43$132
Alliant EnergyIPLWPL
For the year ended202020192020201920202019
Maximum amount outstanding (based on daily outstanding balances)$495$601$8$50$266$195
Average amount outstanding (based on daily outstanding balances)$292$453$—$—$117$93
Weighted average interest rates0.7%2.5%0.5%2.8%0.7%2.4%
Schedule of Long-term Debt Long-term debt, net as of December 31 was as follows (dollars in millions):
20202019
Alliant EnergyIPLWPLAlliant EnergyIPLWPL
Senior Debentures (a):
3.25%, due 2024$500 $500 $— $500 $500 $— 
3.4%, due 2025250 250 — 250 250 — 
5.5%, due 202550 50 — 50 50 — 
4.1%, due 2028500 500 — 500 500 — 
3.6%, due 2029 300 300 — 300 300 — 
2.3%, due 2030 (b)400 400 — — — — 
6.45%, due 2033100 100 — 100 100 — 
6.3%, due 2034125 125 — 125 125 — 
6.25%, due 2039300 300 — 300 300 — 
4.7%, due 2043250 250 — 250 250 — 
3.7%, due 2046 300 300 — 300 300 — 
3.5%, due 2049 300 300 — 300 300 — 
3.65% (Retired in 2020)— — — 200 200 — 
3,375 3,375 — 3,175 3,175 — 
Debentures (a):
2.25%, due 2022250 — 250 250 — 250 
3.05%, due 2027300 — 300 300 — 300 
3%, due 2029 350 — 350 350 — 350 
6.25%, due 2034100 — 100 100 — 100 
6.375%, due 2037300 — 300 300 — 300 
7.6%, due 2038250 — 250 250 — 250 
4.1%, due 2044250 — 250 250 — 250 
3.65%, due 2050 (c)350 — 350 — — — 
4.6% (Retired in 2020)— — — 150 — 150 
2,150 — 2,150 1,950 — 1,950 
Other:
AEF term loan credit agreement through March 2022, 1% at December 31, 2020 (with Alliant Energy as guarantor) (d)300 — — — — — 
Corporate Services 3.45% senior notes, due 2022 (a)75 — — 75 — — 
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)400 — — 400 — — 
AEF 1.4% senior notes, due 2026 (with Alliant Energy as guarantor) (a)(e)200 — — — — — 
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)300 — — 300 — — 
Sheboygan Power, LLC 5.06% senior secured notes, due 2021 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a)31 — — 38 — — 
AEF term loan credit agreement, 2% at December 31, 2019 (with Alliant Energy as guarantor) (Retired in 2020)— — — 300 — — 
Other, 1% at December 31, 2020, due 2021 to 20252 — — 2 — — 
1,308 — — 1,115 — — 
Subtotal6,833 3,375 2,150 6,240 3,175 1,950 
Current maturities(8)— — (657)(200)(150)
Unamortized debt issuance costs(41)(22)(14)(37)(21)(11)
Unamortized debt (discount) and premium, net(15)(8)(6)(13)(7)(6)
Long-term debt, net (f)$6,769 $3,345 $2,130 $5,533 $2,947 $1,783 

(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 June 2020, IPL issued $400 million of 2.3% senior debentures due 2030. The net proceeds from the issuance were used by IPL to retire its $200 million 3.65% senior debentures that would have matured in September 2020 and for general corporate purposes.
(c)In April 2020, WPL issued $350 million of 3.65% debentures due 2050. The net proceeds from the issuance were used by WPL to reduce borrowings under the single credit facility and for general corporate purposes.
(d)In March 2020, AEF entered into a $300 million variable-rate term loan credit agreement and used the borrowings under this agreement to retire its $300 million variable-rate term loan credit agreement that would have expired in April 2020.
(e)In November 2020, AEF issued $200 million of 1.4% senior notes due 2026. The net proceeds from the issuance were used to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(f)There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Maturities of Long-term Debt At December 31, 2020, long-term debt maturities for 2021 through 2025 were as follows (in millions):
20212022202320242025
IPL$—$—$—$500$300
WPL—250———
Corporate Services—75———
AEF83084089—
Alliant Energy$8$633$408$509$300