Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets:
Deferred proceeds	$ 178.3	$ 119.4
Liabilities:
Long-term debt (incl. current maturities)	6,073.7	5,860.8
IPL [Member]
Assets:
Deferred proceeds	178.3	119.4
Liabilities:
Long-term debt (incl. current maturities)	2,852.4	2,691.2
WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,080.0	2,043.7
Commodity Contracts [Member]
Assets:
Derivatives	21.7	28.3
Liabilities:
Derivatives	15.7	23.3
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	11.9	17.7
Liabilities:
Derivatives	7.1	11.2
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	9.8	10.6
Liabilities:
Derivatives	8.6	12.1
Level 1 [Member]
Assets:
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]
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:
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	6,071.6	5,858.4
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities:
Long-term debt (incl. current maturities)	2,852.4	2,691.2
Level 2 [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	2,080.0	2,043.7
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	15.9	8.9
Liabilities:
Derivatives	10.3	16.1
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	7.7	4.0
Liabilities:
Derivatives	3.5	6.5
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.2	4.9
Liabilities:
Derivatives	6.8	9.6
Level 3 [Member]
Assets:
Deferred proceeds	178.3	119.4
Liabilities:
Long-term debt (incl. current maturities)	2.1	2.4
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	178.3	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	5.8	19.4
Liabilities:
Derivatives	5.4	7.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	4.2	13.7
Liabilities:
Derivatives	3.6	4.7
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.6	5.7
Liabilities:
Derivatives	1.8	2.5
Carrying Amount [Member]
Assets:
Deferred proceeds	178.3	119.4
Liabilities:
Long-term debt (incl. current maturities)	5,618.7	5,502.8
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	178.3	119.4
Liabilities:
Long-term debt (incl. current maturities)	2,667.8	2,552.3
Carrying Amount [Member] | WPL [Member]
Liabilities:
Long-term debt (incl. current maturities)	1,835.4	1,834.9
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	21.7	28.3
Liabilities:
Derivatives	15.7	23.3
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	11.9	17.7
Liabilities:
Derivatives	7.1	11.2
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	9.8	10.6
Liabilities:
Derivatives	$ 8.6	$ 12.1