Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Deferred proceeds	$ 208.3	$ 222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	5,872.9	5,447.5
Cumulative preferred stock of IPL	201.4	203.8
IPL [Member]
Assets:
Deferred proceeds	208.3	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,671.5	2,665.7
Cumulative preferred stock of IPL	201.4	203.8
WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,071.8	2,147.9
Commodity Contracts [Member]
Assets:
Derivatives	31.6	25.1
Liabilities and Equity:
Derivatives	42.7	41.7
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	19.0	17.1
Liabilities and Equity:
Derivatives	23.0	19.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	12.6	8.0
Liabilities and Equity:
Derivatives	19.7	22.3
Level 1 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	201.4	203.8
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	201.4	203.8
Level 1 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and Equity:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and Equity:
Derivatives	0.0	0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.0	0.0
Liabilities and Equity:
Derivatives	0.0	0.0
Level 2 [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	5,870.4	5,444.6
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,671.5	2,665.7
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,071.8	2,147.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	5.5	4.1
Liabilities and Equity:
Derivatives	5.9	8.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	2.2	2.0
Liabilities and Equity:
Derivatives	2.1	2.9
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	3.3	2.1
Liabilities and Equity:
Derivatives	3.8	5.6
Level 3 [Member]
Assets:
Deferred proceeds	208.3	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2.5	2.9
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	208.3	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	26.1	21.0
Liabilities and Equity:
Derivatives	36.8	33.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	16.8	15.1
Liabilities and Equity:
Derivatives	20.9	16.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	9.3	5.9
Liabilities and Equity:
Derivatives	15.9	16.7
Carrying Amount [Member]
Assets:
Deferred proceeds	208.3	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	5,483.6	4,866.3
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Deferred proceeds	208.3	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,531.8	2,406.0
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	1,834.1	1,833.4
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	31.6	25.1
Liabilities and Equity:
Derivatives	42.7	41.7
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	19.0	17.1
Liabilities and Equity:
Derivatives	23.0	19.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	12.6	8.0
Liabilities and Equity:
Derivatives	$ 19.7	$ 22.3