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Debt (Long-term Debt) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 3,870.5 | $ 3,802.8 |
Current maturities |  | (313.4) | (183.0) |
Unamortized debt issuance costs |  | (22.3) | (22.4) |
Unamortized debt (discount) and premium, net |  | (12.6) | (13.1) |
Long-term debt, net |  | 3,522.2 | 3,584.3 |
Proceeds from issuance of long-term debt |  | 250.7 | 812.9 | $ 250.0
3.4% senior debenture, due 2025 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Proceeds from issuance of long-term debt |  | 250.0 |  |
Senior Debentures [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | 1,875.0 | 1,775.0 |
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued | [1] | $ 250.0 |  |
Interest rate |  | 3.40% |  |
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 250.0 | $ 250.0 |
Interest rate |  | 4.70% | 4.70% |
Senior Debentures [Member] | 3.3% senior debenture, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 150.0 |
Interest rate |  |  | 3.30% |
Debentures [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 1,550.0 | $ 1,550.0 |
Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 250.0 | $ 250.0 |
Interest rate |  | 4.10% | 4.10% |
Pollution Control Revenue Bonds [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 30.6 |
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 16.0 |
Interest rate |  |  | 5.00% |
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 14.6 |
Interest rate |  |  | 5.375% |
Other Long Term Debt [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 445.5 | $ 447.2 |
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 250.0 | $ 250.0 |
Interest rate |  | 1.00% | 1.00% |
Senior Notes [Member] | 3.45% senior notes, due 2022 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 75.0 | $ 75.0 |
Interest rate |  | 3.45% | 3.45% |
Senior Secured Notes [Member] | 5.06% senior secured notes, due 2016 to 2024 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 56.8 | $ 58.9 |
Interest rate |  | 5.06% | 5.06% |
Other [Member] | Other, 1% at December 31, 2015, due 2016 to 2025 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 3.7 | $ 3.3 |
Interest rate |  | 1.00% | 1.00% |
IPL [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 1,875.0 | $ 1,775.0 |
Current maturities |  | 0.0 | (150.0) |
Unamortized debt issuance costs |  | (11.8) | (10.7) |
Unamortized debt (discount) and premium, net |  | (6.3) | (6.3) |
Long-term debt, net |  | 1,856.9 | 1,608.0 |
Proceeds from issuance of long-term debt |  | 250.0 | 250.0 | 250.0
Repayments of commercial paper |  | 111.0 |  |
IPL [Member] | Senior Debentures [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | 1,875.0 | 1,775.0 |
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued | [1] | $ 250.0 |  |
Interest rate |  | 3.40% |  |
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 250.0 | $ 250.0 |
Interest rate |  | 4.70% | 4.70% |
IPL [Member] | Senior Debentures [Member] | 3.3% senior debenture, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 150.0 |
Interest rate |  |  | 3.30% |
WPL [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 1,550.0 | $ 1,580.6 |
Current maturities |  | 0.0 | (30.6) |
Unamortized debt issuance costs |  | (9.9) | (10.8) |
Unamortized debt (discount) and premium, net |  | (6.2) | (6.7) |
Long-term debt, net |  | 1,533.9 | 1,532.5 |
Proceeds from issuance of long-term debt |  | 0.0 | 250.0 | $ 0.0
WPL [Member] | Debentures [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | 1,550.0 | 1,550.0 |
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 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 issued |  | $ 250.0 | $ 250.0 |
Interest rate |  | 4.10% | 4.10% |
WPL [Member] | Pollution Control Revenue Bonds [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 30.6 |
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 16.0 |
Interest rate |  |  | 5.00% |
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, retired in 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  |  | $ 14.6 |
Interest rate |  |  | 5.375% |
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2015 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt issued |  | $ 60.0 | $ 60.0 |
Interest rate |  | 1.00% | 1.00% |
[1] | In 2015, IPL issued $250.0 million of 3.4% senior debentures due 2025. The proceeds from the issuance were used by IPL to reduce commercial paper classified as long-term debt by $111 million, reduce cash amounts received from its sales of accounts receivable program and for general corporate purposes.

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