Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 16.3	$ 19.3
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,806.5	3,860.5
Cumulative preferred stock		212.6
Cumulative preferred stock	201.8
Derivative liabilities	9.8	33.0
IPL [Member]
Assets:
Derivative assets	9.0	14.5
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,643.8	1,679.9
Cumulative preferred stock	201.8	151.8
Derivative liabilities	2.6	12.6
WPL [Member]
Assets:
Derivative assets	7.3	4.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,691.6	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	7.2	20.4
Commodity Contracts [Member]
Assets:
Derivative assets	23.6	26.2
Capitalization and liabilities:
Derivative liabilities	17.1	40.4
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	12.5	17.5
Capitalization and liabilities:
Derivative liabilities	6.1	16.1
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	11.1	8.7
Capitalization and liabilities:
Derivative liabilities	11.0	24.3
Level 1 [Member]
Assets:
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock		162.3
Cumulative preferred stock	201.8
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	201.8	151.8
Level 1 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	0	10.5
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	0	0
Capitalization and liabilities:
Derivative liabilities	0	0
Level 2 [Member]
Assets:
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	3,806.0	3,860.0
Cumulative preferred stock		50.3
Cumulative preferred stock	0
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,643.8	1,679.9
Cumulative preferred stock	0	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,691.6	1,713.3
Cumulative preferred stock	0	50.3
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	14.4	4.8
Capitalization and liabilities:
Derivative liabilities	12.8	30.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	6.5	3.1
Capitalization and liabilities:
Derivative liabilities	4.4	14.2
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	7.9	1.7
Capitalization and liabilities:
Derivative liabilities	8.4	16.7
Level 3 [Member]
Assets:
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	0.5	0.5
Cumulative preferred stock		0
Cumulative preferred stock	0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	0	0
Level 3 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0	0
Cumulative preferred stock	0	0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivative assets	9.2	21.4
Capitalization and liabilities:
Derivative liabilities	4.3	9.5
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivative assets	6.0	14.4
Capitalization and liabilities:
Derivative liabilities	1.7	1.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivative assets	3.2	7.0
Capitalization and liabilities:
Derivative liabilities	$ 2.6	$ 7.6