Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Level 1 [Member]
Assets:
Deferred proceeds	$ 0.0	$ 0.0
Liabilities and Equity:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock of IPL	208.3	206.6
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0.0	0.0
Liabilities and Equity:
Long-term debt (including current maturities)	0.0	0.0
Cumulative preferred stock of IPL	208.3	206.6
Level 1 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including 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 (including current maturities)	4,452.3	4,332.4
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 (including current maturities)	2,147.2	2,092.7
Cumulative preferred stock of IPL	0.0	0.0
Level 2 [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,854.3	1,793.0
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	1.2	2.5
Liabilities and Equity:
Derivatives	11.6	16.0
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.1	2.0
Liabilities and Equity:
Derivatives	5.5	8.0
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0.1	0.5
Liabilities and Equity:
Derivatives	6.1	8.0
Level 3 [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	3.3	3.7
Cumulative preferred stock of IPL	0.0	0.0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including 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 (including current maturities)	0.0	0.0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	6.5	15.9
Liabilities and Equity:
Derivatives	72.4	48.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	4.7	13.5
Liabilities and Equity:
Derivatives	17.8	15.4
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.8	2.4
Liabilities and Equity:
Derivatives	54.6	33.2
Carrying Value [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	3,836.1	3,835.6
Cumulative preferred stock of IPL	200.0	200.0
Carrying Value [Member] | IPL [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	1,857.4	1,856.9
Cumulative preferred stock of IPL	200.0	200.0
Carrying Value [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,534.2	1,533.9
Carrying Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	7.7	18.4
Liabilities and Equity:
Derivatives	84.0	64.6
Carrying Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	5.8	15.5
Liabilities and Equity:
Derivatives	23.3	23.4
Carrying Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.9	2.9
Liabilities and Equity:
Derivatives	60.7	41.2
Fair Value [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	4,455.6	4,336.1
Cumulative preferred stock of IPL	208.3	206.6
Fair Value [Member] | IPL [Member]
Assets:
Deferred proceeds	154.2	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	2,147.2	2,092.7
Cumulative preferred stock of IPL	208.3	206.6
Fair Value [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,854.3	1,793.0
Fair Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	7.7	18.4
Liabilities and Equity:
Derivatives	84.0	64.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	5.8	15.5
Liabilities and Equity:
Derivatives	23.3	23.4
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.9	2.9
Liabilities and Equity:
Derivatives	$ 60.7	$ 41.2