Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions	Jun. 30, 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	218.3	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	218.3	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)	4,647.2	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)	2,271.3	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)	1,923.5	1,793.0
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	3.2	2.5
Liabilities and Equity:
Derivatives	4.0	16.0
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	1.8	2.0
Liabilities and Equity:
Derivatives	1.8	8.0
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.4	0.5
Liabilities and Equity:
Derivatives	2.2	8.0
Level 3 [Member]
Assets:
Deferred proceeds	74.4	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	74.4	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	33.7	15.9
Liabilities and Equity:
Derivatives	33.1	48.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	26.3	13.5
Liabilities and Equity:
Derivatives	8.0	15.4
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	7.4	2.4
Liabilities and Equity:
Derivatives	25.1	33.2
Carrying Value [Member]
Assets:
Deferred proceeds	74.4	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	3,902.4	3,835.6
Cumulative preferred stock of IPL	200.0	200.0
Carrying Value [Member] | IPL [Member]
Assets:
Deferred proceeds	74.4	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	1,924.8	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)	1,534.6	1,533.9
Carrying Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	36.9	18.4
Liabilities and Equity:
Derivatives	37.1	64.6
Carrying Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	28.1	15.5
Liabilities and Equity:
Derivatives	9.8	23.4
Carrying Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.8	2.9
Liabilities and Equity:
Derivatives	27.3	41.2
Fair Value [Member]
Assets:
Deferred proceeds	74.4	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	4,650.5	4,336.1
Cumulative preferred stock of IPL	218.3	206.6
Fair Value [Member] | IPL [Member]
Assets:
Deferred proceeds	74.4	172.0
Liabilities and Equity:
Long-term debt (including current maturities)	2,271.3	2,092.7
Cumulative preferred stock of IPL	218.3	206.6
Fair Value [Member] | WPL [Member]
Liabilities and Equity:
Long-term debt (including current maturities)	1,923.5	1,793.0
Fair Value [Member] | Commodity Contracts [Member]
Assets:
Derivatives	36.9	18.4
Liabilities and Equity:
Derivatives	37.1	64.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	28.1	15.5
Liabilities and Equity:
Derivatives	9.8	23.4
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.8	2.9
Liabilities and Equity:
Derivatives	$ 27.3	$ 41.2