Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets:
Money market fund investments	$ 174.4	$ 0.0
Deferred proceeds	237.0	119.4
Liabilities:
Long-term debt (incl. current maturities)	7,018.4	5,860.8
IPL [Member]
Assets:
Money market fund investments	173.7	0.0
Deferred proceeds	237.0	119.4
Liabilities:
Long-term debt (incl. current maturities)	3,537.0	2,691.2
WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,314.1	2,043.7
Commodity Contracts [Member]
Assets:
Derivatives	30.3	28.3
Liabilities:
Derivatives	31.8	23.3
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	23.3	17.7
Liabilities:
Derivatives	13.4	11.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	7.0	10.6
Liabilities:
Derivatives	18.4	12.1
Level 1 [Member]
Assets:
Money market fund investments	174.4	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:
Money market fund investments	173.7	0.0
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)	7,016.4	5,858.4
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)	3,537.0	2,691.2
Level 2 [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,314.1	2,043.7
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	8.1	8.9
Liabilities:
Derivatives	31.3	16.1
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	4.8	4.0
Liabilities:
Derivatives	12.9	6.5
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	3.3	4.9
Liabilities:
Derivatives	18.4	9.6
Level 3 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	237.0	119.4
Liabilities:
Long-term debt (incl. current maturities)	2.0	2.4
Level 3 [Member] | IPL [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	237.0	119.4
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	22.2	19.4
Liabilities:
Derivatives	0.5	7.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	18.5	13.7
Liabilities:
Derivatives	0.5	4.7
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	3.7	5.7
Liabilities:
Derivatives	0.0	2.5
Carrying Amount [Member]
Assets:
Money market fund investments	174.4	0.0
Deferred proceeds	237.0	119.4
Liabilities:
Long-term debt (incl. current maturities)	6,191.9	5,502.8
Carrying Amount [Member] | IPL [Member]
Assets:
Money market fund investments	173.7	0.0
Deferred proceeds	237.0	119.4
Liabilities:
Long-term debt (incl. current maturities)	3,146.8	2,552.3
Carrying Amount [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	1,932.3	1,834.9
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	30.3	28.3
Liabilities:
Derivatives	31.8	23.3
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	23.3	17.7
Liabilities:
Derivatives	13.4	11.2
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	7.0	10.6
Liabilities:
Derivatives	$ 18.4	$ 12.1