Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets:
Deferred proceeds	$ 149.0	$ 211.1
Liabilities and Equity:
Long-term debt (including current maturities)	4,771.2	4,799.0
Cumulative preferred stock of IPL	202.6	194.8
IPL [Member]
Assets:
Deferred proceeds	149.0	211.1
Liabilities and Equity:
Long-term debt (including current maturities)	2,342.1	2,352.3
Cumulative preferred stock of IPL	202.6	194.8
WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,790.9	1,807.4
Commodity Contracts [Member]
Assets:
Derivatives	13.3	41.4
Liabilities and Equity:
Derivatives	45.3	28.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	8.8	20.8
Liabilities and Equity:
Derivatives	16.3	8.3
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.5	20.6
Liabilities and Equity:
Derivatives	29.0	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	202.6	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	202.6	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)	4,768.3	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)	2,342.1	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)	1,790.9	1,807.4
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	1.7	4.6
Liabilities and Equity:
Derivatives	0.8	0.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.3	2.8
Liabilities and Equity:
Derivatives	0.5	0.4
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.4	1.8
Liabilities and Equity:
Derivatives	0.3	0.1
Level 3 [Member]
Assets:
Deferred proceeds	149.0	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	149.0	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	11.6	36.8
Liabilities and Equity:
Derivatives	44.5	28.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	7.5	18.0
Liabilities and Equity:
Derivatives	15.8	7.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.1	18.8
Liabilities and Equity:
Derivatives	28.7	20.2
Carrying Amount [Member]
Assets:
Deferred proceeds	149.0	211.1
Liabilities and Equity:
Long-term debt (including current maturities)	4,320.7	4,320.2
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	149.0	211.1
Liabilities and Equity:
Long-term debt (including current maturities)	2,154.0	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)	1,535.5	1,535.2
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	13.3	41.4
Liabilities and Equity:
Derivatives	45.3	28.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	8.8	20.8
Liabilities and Equity:
Derivatives	16.3	8.3
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.5	20.6
Liabilities and Equity:
Derivatives	$ 29.0	$ 20.3