Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets:
Deferred proceeds	$ 66.8	$ 53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,860.5
Cumulative preferred stock of subsidiaries	212.6
IPL [Member]
Assets:
Deferred proceeds	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,679.9
Cumulative preferred stock of subsidiaries	151.8
WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,713.3
Cumulative preferred stock of subsidiaries	60.8
Commodity Contracts [Member]
Assets:
Derivatives	26.2	15.7
Capitalization and liabilities:
Derivatives	40.4	78.0
Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	17.5	10.6
Capitalization and liabilities:
Derivatives	16.1	33.6
Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	8.7	5.1
Capitalization and liabilities:
Derivatives	24.3	44.4
Level 1 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	162.3
Level 1 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	151.8
Level 1 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	10.5
Level 1 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	0	0
Capitalization and liabilities:
Derivatives	0	0
Level 2 [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	3,860.0
Cumulative preferred stock of subsidiaries	50.3
Level 2 [Member] | IPL [Member]
Assets:
Deferred proceeds	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,679.9
Cumulative preferred stock of subsidiaries	0
Level 2 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,713.3
Cumulative preferred stock of subsidiaries	50.3
Level 2 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	4.8	3.4
Capitalization and liabilities:
Derivatives	30.9	64.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	3.1	1.3
Capitalization and liabilities:
Derivatives	14.2	28.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	1.7	2.1
Capitalization and liabilities:
Derivatives	16.7	36.2
Level 3 [Member]
Assets:
Deferred proceeds	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	0.5
Cumulative preferred stock of subsidiaries	0
Level 3 [Member] | IPL [Member]
Assets:
Deferred proceeds	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	0
Level 3 [Member] | WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	0
Cumulative preferred stock of subsidiaries	0
Level 3 [Member] | Commodity Contracts [Member]
Assets:
Derivatives	21.4	12.3
Capitalization and liabilities:
Derivatives	9.5	13.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Assets:
Derivatives	14.4	9.3
Capitalization and liabilities:
Derivatives	1.9	5.0
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Assets:
Derivatives	7.0	3.0
Capitalization and liabilities:
Derivatives	$ 7.6	$ 8.2