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Fair Value Measurements (Fair Value Measurements) (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Assets:    
Deferred proceeds $ 170.0 $ 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 4,881.7 4,799.0
Cumulative preferred stock of IPL 206.2 194.8
IPL [Member]    
Assets:    
Deferred proceeds 170.0 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 2,421.3 2,352.3
Cumulative preferred stock of IPL 206.2 194.8
WPL [Member]    
Liabilities and Equity:    
Long-term debt (including current maturities) 1,823.6 1,807.4
Commodity Contracts [Member]    
Assets:    
Derivatives 37.9 41.4
Liabilities and Equity:    
Derivatives 45.4 28.6
Commodity Contracts [Member] | IPL [Member]    
Assets:    
Derivatives 29.3 20.8
Liabilities and Equity:    
Derivatives 16.3 8.3
Commodity Contracts [Member] | WPL [Member]    
Assets:    
Derivatives 8.6 20.6
Liabilities and Equity:    
Derivatives 29.1 20.3
Level 1 [Member]    
Assets:    
Deferred proceeds 0.0 0.0
Liabilities and Equity:    
Long-term debt (including current maturities) 0.0 0.0
Cumulative preferred stock of IPL 206.2 194.8
Level 1 [Member] | IPL [Member]    
Assets:    
Deferred proceeds 0.0 0.0
Liabilities and Equity:    
Long-term debt (including current maturities) 0.0 0.0
Cumulative preferred stock of IPL 206.2 194.8
Level 1 [Member] | WPL [Member]    
Liabilities and Equity:    
Long-term debt (including current maturities) 0.0 0.0
Level 1 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 0.0 0.0
Liabilities and Equity:    
Derivatives 0.0 0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]    
Assets:    
Derivatives 0.0 0.0
Liabilities and Equity:    
Derivatives 0.0 0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [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) 4,878.8 4,795.7
Cumulative preferred stock of IPL 0.0 0.0
Level 2 [Member] | IPL [Member]    
Assets:    
Deferred proceeds 0.0 0.0
Liabilities and Equity:    
Long-term debt (including current maturities) 2,421.3 2,352.3
Cumulative preferred stock of IPL 0.0 0.0
Level 2 [Member] | WPL [Member]    
Liabilities and Equity:    
Long-term debt (including current maturities) 1,823.6 1,807.4
Level 2 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 1.1 4.6
Liabilities and Equity:    
Derivatives 17.8 0.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]    
Assets:    
Derivatives 1.0 2.8
Liabilities and Equity:    
Derivatives 5.1 0.4
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]    
Assets:    
Derivatives 0.1 1.8
Liabilities and Equity:    
Derivatives 12.7 0.1
Level 3 [Member]    
Assets:    
Deferred proceeds 170.0 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 2.9 3.3
Cumulative preferred stock of IPL 0.0 0.0
Level 3 [Member] | IPL [Member]    
Assets:    
Deferred proceeds 170.0 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 0.0 0.0
Cumulative preferred stock of IPL 0.0 0.0
Level 3 [Member] | WPL [Member]    
Liabilities and Equity:    
Long-term debt (including current maturities) 0.0 0.0
Level 3 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 36.8 36.8
Liabilities and Equity:    
Derivatives 27.6 28.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]    
Assets:    
Derivatives 28.3 18.0
Liabilities and Equity:    
Derivatives 11.2 7.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]    
Assets:    
Derivatives 8.5 18.8
Liabilities and Equity:    
Derivatives 16.4 20.2
Carrying Amount [Member]    
Assets:    
Deferred proceeds 170.0 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 4,359.5 4,320.2
Cumulative preferred stock of IPL 200.0 200.0
Carrying Amount [Member] | IPL [Member]    
Assets:    
Deferred proceeds 170.0 211.1
Liabilities and Equity:    
Long-term debt (including current maturities) 2,194.5 2,153.5
Cumulative preferred stock of IPL 200.0 200.0
Carrying Amount [Member] | WPL [Member]    
Liabilities and Equity:    
Long-term debt (including current maturities) 1,535.9 1,535.2
Carrying Amount [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 37.9 41.4
Liabilities and Equity:    
Derivatives 45.4 28.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]    
Assets:    
Derivatives 29.3 20.8
Liabilities and Equity:    
Derivatives 16.3 8.3
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]    
Assets:    
Derivatives 8.6 20.6
Liabilities and Equity:    
Derivatives $ 29.1 $ 20.3