Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Money market fund investments	$ 228.0	$ 0.0
Deferred proceeds	243.7	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	6,060.3	5,447.5
Cumulative preferred stock of IPL	200.0	203.8
IPL [Member]
Assets:
Money market fund investments	228.0	0.0
Deferred proceeds	243.7	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,916.2	2,665.7
Cumulative preferred stock of IPL	200.0	203.8
WPL [Member]
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,020.9	2,147.9
Commodity Contracts [Member]
Assets:
Derivatives	48.0	25.1
Liabilities and Equity:
Derivatives	22.0	41.7
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	29.1	17.1
Liabilities and Equity:
Derivatives	9.7	19.4
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	18.9	8.0
Liabilities and Equity:
Derivatives	12.3	22.3
Level 1 [Member]
Assets:
Money market fund investments	228.0	0.0
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	200.0	203.8
Level 1 [Member] | IPL [Member]
Assets:
Money market fund investments	228.0	0.0
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	0.0	0.0
Cumulative preferred stock of IPL	200.0	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:
Money market fund investments	0.0	0.0
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	6,057.8	5,444.6
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | IPL [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,916.2	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)	2,020.9	2,147.9
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	16.5	4.1
Liabilities and Equity:
Derivatives	14.7	8.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	6.7	2.0
Liabilities and Equity:
Derivatives	5.5	2.9
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	9.8	2.1
Liabilities and Equity:
Derivatives	9.2	5.6
Level 3 [Member]
Assets:
Money market fund investments	0.0	0.0
Deferred proceeds	243.7	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:
Money market fund investments	0.0	0.0
Deferred proceeds	243.7	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	31.5	21.0
Liabilities and Equity:
Derivatives	7.3	33.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	22.4	15.1
Liabilities and Equity:
Derivatives	4.2	16.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	9.1	5.9
Liabilities and Equity:
Derivatives	3.1	16.7
Carrying Amount [Member]
Assets:
Money market fund investments	228.0	0.0
Deferred proceeds	243.7	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	5,754.3	4,866.3
Cumulative preferred stock of IPL	200.0	200.0
Carrying Amount [Member] | IPL [Member]
Assets:
Money market fund investments	228.0	0.0
Deferred proceeds	243.7	222.1
Liabilities and Equity:
Long-term debt (incl. current maturities)	2,801.8	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)	1,834.5	1,833.4
Carrying Amount [Member] | Commodity Contracts [Member]
Assets:
Derivatives	48.0	25.1
Liabilities and Equity:
Derivatives	22.0	41.7
Carrying Amount [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	29.1	17.1
Liabilities and Equity:
Derivatives	9.7	19.4
Carrying Amount [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	18.9	8.0
Liabilities and Equity:
Derivatives	$ 12.3	$ 22.3