Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets:
Money market fund investments	$ 161.8	$ 0.0
Deferred proceeds	214.6	119.4
Liabilities:
Long-term debt (incl. current maturities)	6,790.9	5,860.8
IPL [Member]
Assets:
Deferred proceeds	214.6	119.4
Liabilities:
Long-term debt (incl. current maturities)	3,135.3	2,691.2
WPL [Member]
Assets:
Money market fund investments	160.4	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,498.2	2,043.7
Commodity Contracts [Member]
Assets:
Derivatives	21.9	28.3
Liabilities:
Derivatives	28.0	23.3
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	13.9	17.7
Liabilities:
Derivatives	12.2	11.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.0	10.6
Liabilities:
Derivatives	15.8	12.1
Level 1 [Member]
Assets:
Money market fund investments	161.8	0.0
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | IPL [Member]
Assets:
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	160.4	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)	6,788.8	5,858.4
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	3,135.3	2,691.2
Level 2 [Member] | WPL [Member]
Assets:
Money market fund investments	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,498.2	2,043.7
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8.2	8.9
Liabilities:
Derivatives	27.5	16.1
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	4.1	4.0
Liabilities:
Derivatives	11.7	6.5
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	4.1	4.9
Liabilities:
Derivatives	15.8	9.6
Level 3 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	214.6	119.4
Liabilities:
Long-term debt (incl. current maturities)	2.1	2.4
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	214.6	119.4
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	13.7	19.4
Liabilities:
Derivatives	0.5	7.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	9.8	13.7
Liabilities:
Derivatives	0.5	4.7
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	3.9	5.7
Liabilities:
Derivatives	0.0	2.5
Carrying Amount [Member]
Assets:
Money market fund investments	161.8	0.0
Deferred proceeds	214.6	119.4
Liabilities:
Long-term debt (incl. current maturities)	6,144.9	5,502.8
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	214.6	119.4
Liabilities:
Long-term debt (incl. current maturities)	2,850.5	2,552.3
Carrying Amount [Member] | WPL [Member]
Assets:
Money market fund investments	160.4	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,181.8	1,834.9
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	21.9	28.3
Liabilities:
Derivatives	28.0	23.3
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	13.9	17.7
Liabilities:
Derivatives	12.2	11.2
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.0	10.6
Liabilities:
Derivatives	$ 15.8	$ 12.1