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Debt (Long-term Debt) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 6,240.3 | $ 5,546.7
Current maturities |  | (657.2) | (256.5)
Unamortized debt issuance costs |  | (36.9) | (32.1)
Unamortized debt (discount) and premium, net |  | (13.2) | (11.8)
Long-term debt, net | [1] | 5,533.0 | 5,246.3
Senior Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 3,175.0 | 2,575.0
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 200.0 | $ 200.0
Interest rate |  | 3.65% | 3.65%
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 500.0 | $ 500.0
Interest rate |  | 3.25% | 3.25%
Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 3.40% | 3.40%
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 50.0 | $ 50.0
Interest rate |  | 5.50% | 5.50%
Senior Debentures [Member] | 4.1% senior debenture, due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 500.0 | $ 500.0
Interest rate |  | 4.10% | 4.10%
Senior Debentures [Member] | 3.6% senior debenture, due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[3] | $ 300.0 | $ 0.0
Interest rate |  | 3.60% |
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 6.45% | 6.45%
Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 125.0 | $ 125.0
Interest rate |  | 6.30% | 6.30%
Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 6.25% | 6.25%
Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 4.70% | 4.70%
Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 3.70% | 3.70%
Senior Debentures [Member] | 3.5% senior debenture, due 2049 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[3] | $ 300.0 | $ 0.0
Interest rate |  | 3.50% |
Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 1,950.0 | 1,850.0
Debentures [Member] | 4.6% debenture, due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 150.0 | $ 150.0
Interest rate |  | 4.60% | 4.60%
Debentures [Member] | 2.25% debenture, due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 2.25% | 2.25%
Debentures [Member] | 3.05% debenture, due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 3.05% | 3.05%
Debentures [Member] | 3% debenture, due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[4] | $ 350.0 | $ 0.0
Interest rate |  | 3.00% |
Debentures [Member] | 6.25% debenture, due 2034 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 6.25% | 6.25%
Debentures [Member] | 6.375% debenture, due 2037 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 6.375% | 6.375%
Debentures [Member] | 7.6% debenture, due 2038 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 7.60% | 7.60%
Debentures [Member] | 4.1% debenture, due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 4.10% | 4.10%
Debentures [Member] | 5% debenture, retired in 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 0.0 | $ 250.0
Interest rate |  |  | 5.00%
Other Long Term Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | 1,115.3 | $ 1,121.7
Other [Member] | Other, 1% at December 31, 2019, due 2020 to 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 2.1 | 2.4
Interest rate |  | 1.00% |
IPL [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 3,175.0 | 2,575.0
Current maturities |  | (200.0) | 0.0
Unamortized debt issuance costs |  | (21.0) | (17.2)
Unamortized debt (discount) and premium, net |  | (6.7) | (5.5)
Long-term debt, net | [1] | 2,947.3 | 2,552.3
IPL [Member] | Senior Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 3,175.0 | 2,575.0
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 200.0 | $ 200.0
Interest rate |  | 3.65% | 3.65%
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 500.0 | $ 500.0
Interest rate |  | 3.25% | 3.25%
IPL [Member] | Senior Debentures [Member] | 3.4% senior debenture, due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 3.40% | 3.40%
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 50.0 | $ 50.0
Interest rate |  | 5.50% | 5.50%
IPL [Member] | Senior Debentures [Member] | 4.1% senior debenture, due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 500.0 | $ 500.0
Interest rate |  | 4.10% | 4.10%
IPL [Member] | Senior Debentures [Member] | 3.6% senior debenture, due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[3] | $ 300.0 | $ 0.0
Interest rate |  | 3.60% |
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 6.45% | 6.45%
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 125.0 | $ 125.0
Interest rate |  | 6.30% | 6.30%
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 6.25% | 6.25%
IPL [Member] | Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 4.70% | 4.70%
IPL [Member] | Senior Debentures [Member] | 3.7% senior debenture, due 2046 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 3.70% | 3.70%
IPL [Member] | Senior Debentures [Member] | 3.5% senior debenture, due 2049 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[3] | $ 300.0 | $ 0.0
Interest rate |  | 3.50% |
WPL [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 1,950.0 | 1,850.0
Current maturities |  | (150.0) | (250.0)
Unamortized debt issuance costs |  | (11.3) | (9.6)
Unamortized debt (discount) and premium, net |  | (6.0) | (5.5)
Long-term debt, net | [1] | 1,782.7 | 1,584.9
WPL [Member] | Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 1,950.0 | 1,850.0
WPL [Member] | Debentures [Member] | 4.6% debenture, due 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 150.0 | $ 150.0
Interest rate |  | 4.60% | 4.60%
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 2.25% | 2.25%
WPL [Member] | Debentures [Member] | 3.05% debenture, due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 3.05% | 3.05%
WPL [Member] | Debentures [Member] | 3% debenture, due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2],[4] | $ 350.0 | $ 0.0
Interest rate |  | 3.00% |
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 6.25% | 6.25%
WPL [Member] | Debentures [Member] | 6.375% debenture, due 2037 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 6.375% | 6.375%
WPL [Member] | Debentures [Member] | 7.6% debenture, due 2038 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 7.60% | 7.60%
WPL [Member] | Debentures [Member] | 4.1% debenture, due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 4.10% | 4.10%
WPL [Member] | Debentures [Member] | 5% debenture, retired in 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 0.0 | $ 250.0
Interest rate |  |  | 5.00%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through April 2020 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 300.0 | $ 300.0
Interest rate |  | 2.00% |
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 3.75% senior notes, due 2023 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 400.0 | $ 400.0
Interest rate |  | 3.75% | 3.75%
Alliant Energy Finance, LLC [Member] | Senior Notes [Member] | 4.25% senior notes, due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 300.0 | $ 300.0
Interest rate |  | 4.25% | 4.25%
Corporate Services [Member] | Senior Notes [Member] | 3.45% senior notes, due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 75.0 | $ 75.0
Interest rate |  | 3.45% | 3.45%
Sheboygan Power, LLC [Member] | Senior Secured Notes [Member] | 5.06% senior secured notes, due 2020 to 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 38.2 | $ 44.3
Interest rate |  | 5.06% | 5.06%
[1] | There were no significant sinking fund requirements related to the outstanding long-term debt.
[2] | 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.
[3] | In April 2019, IPL issued $300 million of 3.6% senior debentures due 2029. In September 2019, IPL issued $300 million of 3.5% senior debentures due 2049. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of IPL’s wind projects.
[4] | In June 2019, WPL issued $350 million of 3% debentures due 2029. The net proceeds from the issuance were used by WPL to reduce its outstanding commercial paper and retire its $250 million 5% debentures that matured in July 2019.

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