Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets:
Money market fund investments	$ 198.3	$ 5.4
Deferred proceeds	194.1	187.7
Liabilities:
Long-term debt (incl. current maturities)	7,769.8	6,919.9
IPL [Member]
Assets:
Money market fund investments	198.3	5.4
Deferred proceeds	194.1	187.7
Liabilities:
Long-term debt (incl. current maturities)	3,924.7	3,489.1
WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,657.2	2,268.2
Commodity Contracts [Member]
Assets:
Derivatives	42.4	26.8
Liabilities:
Derivatives	46.2	37.6
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	37.0	22.3
Liabilities:
Derivatives	20.3	17.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	5.4	4.5
Liabilities:
Derivatives	25.9	20.2
Level 1 [Member]
Assets:
Money market fund investments	198.3	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	198.3	5.4
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | WPL [Member]
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)	7,768.1	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)	3,924.7	3,489.1
Level 2 [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,657.2	2,268.2
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.6	4.8
Liabilities:
Derivatives	44.8	36.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	3.3	2.8
Liabilities:
Derivatives	19.0	16.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.3	2.0
Liabilities:
Derivatives	25.8	20.2
Level 3 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	194.1	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	194.1	187.7
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 3 [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	37.8	22.0
Liabilities:
Derivatives	1.4	0.8
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	33.7	19.5
Liabilities:
Derivatives	1.3	0.8
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.1	2.5
Liabilities:
Derivatives	0.1	0.0
Carrying Amount [Member]
Assets:
Money market fund investments	198.3	5.4
Deferred proceeds	194.1	187.7
Liabilities:
Long-term debt (incl. current maturities)	6,579.8	6,190.2
Carrying Amount [Member] | IPL [Member]
Assets:
Money market fund investments	198.3	5.4
Deferred proceeds	194.1	187.7
Liabilities:
Long-term debt (incl. current maturities)	3,343.5	3,147.3
Carrying Amount [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,129.4	1,932.7
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	42.4	26.8
Liabilities:
Derivatives	46.2	37.6
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	37.0	22.3
Liabilities:
Derivatives	20.3	17.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	5.4	4.5
Liabilities:
Derivatives	$ 25.9	$ 20.2