Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets:
Money market fund investments	$ 41.2	$ 5.4
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	6,918.1	6,919.9
IPL [Member]
Assets:
Money market fund investments	21.2	5.4
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	3,452.1	3,489.1
WPL [Member]
Assets:
Money market fund investments	20.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,309.8	2,268.2
Commodity Contracts [Member]
Assets:
Derivatives	18.7	26.8
Liabilities:
Derivatives	51.1	37.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	16.5	22.3
Liabilities:
Derivatives	22.4	17.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.2	4.5
Liabilities:
Derivatives	28.7	20.2
Level 1 [Member]
Assets:
Money market fund investments	41.2	5.4
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | IPL [Member]
Assets:
Money market fund investments	21.2	5.4
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | WPL [Member]
Assets:
Money market fund investments	20.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities:
Derivatives	0.0	0.0
Level 2 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	6,916.4	6,917.9
Level 2 [Member] | IPL [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	3,452.1	3,489.1
Level 2 [Member] | WPL [Member]
Assets:
Money market fund investments	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,309.8	2,268.2
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.0	4.8
Liabilities:
Derivatives	50.6	36.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	2.5	2.8
Liabilities:
Derivatives	21.9	16.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.5	2.0
Liabilities:
Derivatives	28.7	20.2
Level 3 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	1.7	2.0
Level 3 [Member] | IPL [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 3 [Member] | WPL [Member]
Assets:
Money market fund investments	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	14.7	22.0
Liabilities:
Derivatives	0.5	0.8
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	14.0	19.5
Liabilities:
Derivatives	0.5	0.8
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.7	2.5
Liabilities:
Derivatives	0.0	0.0
Carrying Amount [Member]
Assets:
Money market fund investments	41.2	5.4
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	6,191.1	6,190.2
Carrying Amount [Member] | IPL [Member]
Assets:
Money market fund investments	21.2	5.4
Deferred proceeds	188.0	187.7
Liabilities:
Long-term debt (incl. current maturities)	3,147.9	3,147.3
Carrying Amount [Member] | WPL [Member]
Assets:
Money market fund investments	20.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	1,933.1	1,932.7
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	18.7	26.8
Liabilities:
Derivatives	51.1	37.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	16.5	22.3
Liabilities:
Derivatives	22.4	17.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	2.2	4.5
Liabilities:
Derivatives	$ 28.7	$ 20.2