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Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Assets: |  |
Deferred proceeds | $ 211.1 | $ 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 4,799.0 | 4,336.1
IPL's cumulative preferred stock | 194.8 | 206.6
Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 41.4 | 18.4
Liabilities and equity: |  |
Derivatives | 28.6 | 64.6
Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
IPL's cumulative preferred stock | 194.8 | 206.6
Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0.0 | 0.0
Liabilities and equity: |  |
Derivatives | 0.0 | 0.0
Level 2 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 4,795.7 | 4,332.4
IPL's cumulative preferred stock | 0.0 | 0.0
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 4.6 | 2.5
Liabilities and equity: |  |
Derivatives | 0.5 | 16.0
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 211.1 | 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 3.3 | 3.7
IPL's cumulative preferred stock | 0.0 | 0.0
Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 36.8 | 15.9
Liabilities and equity: |  |
Derivatives | 28.1 | 48.6
Carrying Amount [Member] |  |
Assets: |  |
Deferred proceeds | 211.1 | 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 4,320.2 | 3,835.6
IPL's cumulative preferred stock | 200.0 | 200.0
Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 41.4 | 18.4
Liabilities and equity: |  |
Derivatives | 28.6 | 64.6
IPL [Member] |  |
Assets: |  |
Deferred proceeds | 211.1 | 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,352.3 | 2,092.7
IPL's cumulative preferred stock | 194.8 | 206.6
IPL [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 20.8 | 15.5
Liabilities and equity: |  |
Derivatives | 8.3 | 23.4
IPL [Member] | Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
IPL's cumulative preferred stock | 194.8 | 206.6
IPL [Member] | Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0.0 | 0.0
Liabilities and equity: |  |
Derivatives | 0.0 | 0.0
IPL [Member] | Level 2 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,352.3 | 2,092.7
IPL's cumulative preferred stock | 0.0 | 0.0
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 2.8 | 2.0
Liabilities and equity: |  |
Derivatives | 0.4 | 8.0
IPL [Member] | Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 211.1 | 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
IPL's cumulative preferred stock | 0.0 | 0.0
IPL [Member] | Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 18.0 | 13.5
Liabilities and equity: |  |
Derivatives | 7.9 | 15.4
IPL [Member] | Carrying Amount [Member] |  |
Assets: |  |
Deferred proceeds | 211.1 | 172.0
Liabilities and equity: |  |
Long-term debt (including current maturities) | 2,153.5 | 1,856.9
IPL's cumulative preferred stock | 200.0 | 200.0
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 20.8 | 15.5
Liabilities and equity: |  |
Derivatives | 8.3 | 23.4
WPL [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 1,807.4 | 1,793.0
WPL [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 20.6 | 2.9
Liabilities and equity: |  |
Derivatives | 20.3 | 41.2
WPL [Member] | Level 1 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
WPL [Member] | Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 0.0 | 0.0
Liabilities and equity: |  |
Derivatives | 0.0 | 0.0
WPL [Member] | Level 2 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 1,807.4 | 1,793.0
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 1.8 | 0.5
Liabilities and equity: |  |
Derivatives | 0.1 | 8.0
WPL [Member] | Level 3 [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
WPL [Member] | Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 18.8 | 2.4
Liabilities and equity: |  |
Derivatives | 20.2 | 33.2
WPL [Member] | Carrying Amount [Member] |  |
Liabilities and equity: |  |
Long-term debt (including current maturities) | 1,535.2 | 1,533.9
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivatives | 20.6 | 2.9
Liabilities and equity: |  |
Derivatives | $ 20.3 | $ 41.2

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