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Fair Value Measurements (Recurring Fair Value Measurements) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Assets:    
Deferred proceeds $ 222.1 $ 211.1
Liabilities and equity:    
Long-term debt (including current maturities) 5,447.5 4,799.0
IPL's cumulative preferred stock 203.8 194.8
Commodity Contracts [Member]    
Assets:    
Derivatives 25.1 41.4
Liabilities and equity:    
Derivatives 41.7 28.6
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 203.8 194.8
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) 5,444.6 4,795.7
IPL's cumulative preferred stock 0.0 0.0
Level 2 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 4.1 4.6
Liabilities and equity:    
Derivatives 8.5 0.5
Level 3 [Member]    
Assets:    
Deferred proceeds 222.1 211.1
Liabilities and equity:    
Long-term debt (including current maturities) 2.9 3.3
IPL's cumulative preferred stock 0.0 0.0
Level 3 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 21.0 36.8
Liabilities and equity:    
Derivatives 33.2 28.1
Carrying Amount [Member]    
Assets:    
Deferred proceeds 222.1 211.1
Liabilities and equity:    
Long-term debt (including current maturities) 4,866.3 4,320.2
IPL's cumulative preferred stock 200.0 200.0
Carrying Amount [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 25.1 41.4
Liabilities and equity:    
Derivatives 41.7 28.6
IPL [Member]    
Assets:    
Deferred proceeds 222.1 211.1
Liabilities and equity:    
Long-term debt (including current maturities) 2,665.7 2,352.3
IPL's cumulative preferred stock 203.8 194.8
IPL [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 17.1 20.8
Liabilities and equity:    
Derivatives 19.4 8.3
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 203.8 194.8
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) 2,665.7 2,352.3
IPL's cumulative preferred stock 0.0 0.0
IPL [Member] | Level 2 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 2.0 2.8
Liabilities and equity:    
Derivatives 2.9 0.4
IPL [Member] | Level 3 [Member]    
Assets:    
Deferred proceeds 222.1 211.1
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 15.1 18.0
Liabilities and equity:    
Derivatives 16.5 7.9
IPL [Member] | Carrying Amount [Member]    
Assets:    
Deferred proceeds 222.1 211.1
Liabilities and equity:    
Long-term debt (including current maturities) 2,406.0 2,153.5
IPL's cumulative preferred stock 200.0 200.0
IPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 17.1 20.8
Liabilities and equity:    
Derivatives 19.4 8.3
WPL [Member]    
Liabilities and equity:    
Long-term debt (including current maturities) 2,147.9 1,807.4
WPL [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 8.0 20.6
Liabilities and equity:    
Derivatives 22.3 20.3
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) 2,147.9 1,807.4
WPL [Member] | Level 2 [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 2.1 1.8
Liabilities and equity:    
Derivatives 5.6 0.1
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 5.9 18.8
Liabilities and equity:    
Derivatives 16.7 20.2
WPL [Member] | Carrying Amount [Member]    
Liabilities and equity:    
Long-term debt (including current maturities) 1,833.4 1,535.2
WPL [Member] | Carrying Amount [Member] | Commodity Contracts [Member]    
Assets:    
Derivatives 8.0 20.6
Liabilities and equity:    
Derivatives $ 22.3 $ 20.3