Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Money market fund investments	$ 21.3
Deferred proceeds	32.9	53.7
Long-term debt (including current maturities), Fair Value	3,317.0
Cumulative preferred stock of subsidiaries, Fair Value	220.6
IPL [Member]
Money market fund investments	21.3
Deferred proceeds	32.9	53.7
Long-term debt (including current maturities), Fair Value	1,582.0
Cumulative preferred stock, Fair Value	161.4
WPL [Member]
Long-term debt (including current maturities), Fair Value	1,409.0
Cumulative preferred stock, Fair Value	59.2
Cumulative preferred stock of subsidiaries, Fair Value	59.2
Commodity Contracts [Member]
Derivatives, Assets	10.4	15.7
Derivatives, Capitalization and liabilities	83.2	78.0
Commodity Contracts [Member] | IPL [Member]
Derivatives, Assets	5.9	10.6
Derivatives, Capitalization and liabilities	38.1	33.6
Commodity Contracts [Member] | WPL [Member]
Derivatives, Assets	4.5	5.1
Derivatives, Capitalization and liabilities	45.1	44.4
Level 1 [Member]
Money market fund investments	21.3
Cumulative preferred stock of subsidiaries, Fair Value	171.1
Level 1 [Member] | IPL [Member]
Money market fund investments	21.3
Cumulative preferred stock, Fair Value	161.4
Level 1 [Member] | WPL [Member]
Cumulative preferred stock, Fair Value	9.7
Cumulative preferred stock of subsidiaries, Fair Value	9.7
Level 2 [Member]
Long-term debt (including current maturities), Fair Value	3,316.5
Cumulative preferred stock of subsidiaries, Fair Value	49.5
Level 2 [Member] | IPL [Member]
Long-term debt (including current maturities), Fair Value	1,582.0
Level 2 [Member] | WPL [Member]
Long-term debt (including current maturities), Fair Value	1,409.0
Cumulative preferred stock, Fair Value	49.5
Cumulative preferred stock of subsidiaries, Fair Value	49.5
Level 2 [Member] | Commodity Contracts [Member]
Derivatives, Assets	4.9	3.4
Derivatives, Capitalization and liabilities	62.6	64.8
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Derivatives, Assets	2.3	1.3
Derivatives, Capitalization and liabilities	23.1	28.6
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Derivatives, Assets	2.6	2.1
Derivatives, Capitalization and liabilities	39.5	36.2
Level 3 [Member]
Deferred proceeds	32.9	53.7
Long-term debt (including current maturities), Fair Value	0.5
Level 3 [Member] | IPL [Member]
Deferred proceeds	32.9	53.7
Level 3 [Member] | Commodity Contracts [Member]
Derivatives, Assets	5.5	12.3
Derivatives, Capitalization and liabilities	20.6	13.2
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Derivatives, Assets	3.6	9.3
Derivatives, Capitalization and liabilities	15.0	5.0
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Derivatives, Assets	1.9	3.0
Derivatives, Capitalization and liabilities	$ 5.6	$ 8.2