Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets:
Deferred proceeds	$ 115.3	$ 211.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	4,896.2	4,799.0
Cumulative preferred stock of IPL	202.3	194.8
IPL [Member]
Assets:
Deferred proceeds	115.3	211.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,430.1	2,352.3
Cumulative preferred stock of IPL	202.3	194.8
WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	1,829.3	1,807.4
Commodity Contracts [Member]
Assets:
Derivatives	29.4	41.4
Liabilities and Equity:
Derivatives	45.1	28.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	21.1	20.8
Liabilities and Equity:
Derivatives	18.7	8.3
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.3	20.6
Liabilities and Equity:
Derivatives	26.4	20.3
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	202.3	194.8
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	202.3	194.8
Level 1 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. 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 (incl. current maturities)	4,893.3	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 (incl. current maturities)	2,430.1	2,352.3
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	1,829.3	1,807.4
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	2.9	4.6
Liabilities and Equity:
Derivatives	14.9	0.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.6	2.8
Liabilities and Equity:
Derivatives	4.5	0.4
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.3	1.8
Liabilities and Equity:
Derivatives	10.4	0.1
Level 3 [Member]
Assets:
Deferred proceeds	115.3	211.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2.9	3.3
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	115.3	211.1
Liabilities and Equity:
Long-term debt (incl. 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 (incl. current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	26.5	36.8
Liabilities and Equity:
Derivatives	30.2	28.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	19.5	18.0
Liabilities and Equity:
Derivatives	14.2	7.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	7.0	18.8
Liabilities and Equity:
Derivatives	16.0	20.2
Carrying Amount [Member]
Assets:
Deferred proceeds	115.3	211.1
Liabilities and Equity:
Long-term debt (including current maturities)	4,360.3	4,320.2
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	115.3	211.1
Liabilities and Equity:
Long-term debt (including current maturities)	2,195.0	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)	1,536.2	1,535.2
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	29.4	41.4
Liabilities and Equity:
Derivatives	45.1	28.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	21.1	20.8
Liabilities and Equity:
Derivatives	18.7	8.3
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.3	20.6
Liabilities and Equity:
Derivatives	$ 26.4	$ 20.3