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Debt (Long-term Debt) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 4,357.1 | $ 3,870.5
Current maturities |  | (4.6) | (313.4)
Unamortized debt issuance costs |  | (23.4) | (22.3)
Unamortized debt (discount) and premium, net |  | (13.5) | (12.6)
Long-term debt, net | [1] | 4,315.6 | 3,522.2
Senior Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 2,175.0 | 1,875.0
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 5.875% | 5.875%
Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 7.25% | 7.25%
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] | $ 250.0 | $ 250.0
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] | 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],[3] | $ 300.0 |
Interest rate |  | 3.70% |
Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 1,550.0 | $ 1,550.0
Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 5.00% | 5.00%
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] | 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%
Other Long Term Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 632.1 | $ 445.5
Term Loan Credit Agreement [Member] | Term loan credit agreement retired in 2016, 1% at December 31, 2015 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [4] |  | $ 250.0
Interest rate |  |  | 1.00%
Other [Member] | Other, 1% at December 31, 2016, due 2017 to 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 3.3 | $ 3.7
Interest rate |  | 1.00% | 1.00%
IPL [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 2,175.0 | $ 1,875.0
Current maturities |  | 0.0 | 0.0
Unamortized debt issuance costs |  | (13.7) | (11.8)
Unamortized debt (discount) and premium, net |  | (7.8) | (6.3)
Long-term debt, net | [1] | 2,153.5 | 1,856.9
Repayments of commercial paper |  | 100.0 |
IPL [Member] | Senior Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 2,175.0 | 1,875.0
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 100.0 | $ 100.0
Interest rate |  | 5.875% | 5.875%
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 7.25% | 7.25%
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] | $ 250.0 | $ 250.0
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] | 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],[3] | $ 300.0 |
Proceeds from the issuance of debt |  | $ 300.0 |
Interest rate |  | 3.70% |
WPL [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt |  | $ 1,550.0 | $ 1,550.0
Current maturities |  | 0.0 | 0.0
Unamortized debt issuance costs |  | (9.1) | (9.9)
Unamortized debt (discount) and premium, net |  | (5.7) | (6.2)
Long-term debt, net | [1] | 1,535.2 | 1,533.9
WPL [Member] | Debentures [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | 1,550.0 | 1,550.0
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 250.0 | $ 250.0
Interest rate |  | 5.00% | 5.00%
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] | 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%
Alliant Energy Finance, LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2018, 1% at December 31, 2016 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [4],[5] | $ 500.0 |
Proceeds from the issuance of debt |  | $ 500.0 |
Interest rate |  | 1.00% |
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 2017 to 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [2] | $ 53.8 | $ 56.8
Interest rate |  | 5.06% | 5.06%
Franklin County Holdings LLC [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement retired in 2016, 1% at December 31, 2015 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | [4] |  | $ 60.0
Interest rate |  |  | 1.00%
[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 2016, IPL issued $300 million of 3.7% senior debentures due 2046. The proceeds from the issuance were used by IPL to reduce cash amounts received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt by $100 million and for general corporate purposes.
[4] | In 2016, AEF entered into a $500 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement to retire Alliant Energy’s and Franklin County Holdings LLC’s variable-rate term loan credit agreements that matured in 2016, reduce outstanding commercial paper at Alliant Energy and for general corporate purposes.
[5] | Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.

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