Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets:
Deferred proceeds	$ 120.9	$ 222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	5,369.1	5,447.5
Cumulative preferred stock of IPL	198.4	203.8
IPL [Member]
Assets:
Deferred proceeds	120.9	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,635.7	2,665.7
Cumulative preferred stock of IPL	198.4	203.8
WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,102.7	2,147.9
Commodity Contracts [Member]
Assets:
Derivatives	14.1	25.1
Liabilities and Equity:
Derivatives	46.7	41.7
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	7.9	17.1
Liabilities and Equity:
Derivatives	24.7	19.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	6.2	8.0
Liabilities and Equity:
Derivatives	22.0	22.3
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	198.4	203.8
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	198.4	203.8
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)	5,366.6	5,444.6
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)	2,635.7	2,665.7
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,102.7	2,147.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.1	4.1
Liabilities and Equity:
Derivatives	7.3	8.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.7	2.0
Liabilities and Equity:
Derivatives	3.1	2.9
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.4	2.1
Liabilities and Equity:
Derivatives	4.2	5.6
Level 3 [Member]
Assets:
Deferred proceeds	120.9	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2.5	2.9
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	120.9	222.1
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	10.0	21.0
Liabilities and Equity:
Derivatives	39.4	33.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	6.2	15.1
Liabilities and Equity:
Derivatives	21.6	16.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	3.8	5.9
Liabilities and Equity:
Derivatives	17.8	16.7
Carrying Amount [Member]
Assets:
Deferred proceeds	120.9	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	4,912.5	4,866.3
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	120.9	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,452.2	2,406.0
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	1,833.7	1,833.4
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	14.1	25.1
Liabilities and Equity:
Derivatives	46.7	41.7
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	7.9	17.1
Liabilities and Equity:
Derivatives	24.7	19.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	6.2	8.0
Liabilities and Equity:
Derivatives	$ 22.0	$ 22.3