Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Money market fund investments	$ 21.3
Long-term debt (including current maturities), Fair Value	3,317.0
Cumulative preferred stock of subsidiaries, Fair Value	220.6
Cumulative preferred stock of subsidiaries, Carrying Amount	145.1	145.1
Cumulative preferred stock of subsidiaries, Carrying Amount	60.0	60.0
IPL [Member]
Money market fund investments	21.3
Deferred proceeds (sales of receivables)	34.6	55.3
Long-term debt (including current maturities), Fair Value	1,582.0
Cumulative preferred stock of subsidiaries, Carrying Amount	145.1	145.1
Cumulative preferred stock of subsidiaries, Fair Value	161.4
WPL [Member]
Long-term debt (including current maturities), Fair Value	1,409.0
Cumulative preferred stock of subsidiaries, Carrying Amount	60.0	60.0
Cumulative preferred stock of subsidiaries, Fair Value	59.2
Carrying Amount [Member]
Money market fund investments	21.3
Derivative assets	10.4	15.7
Deferred proceeds (sales of receivables)	32.9	53.7
Long-term debt (including current maturities), Carrying Amount	2,729.6	2,704.5
Cumulative preferred stock of subsidiaries, Carrying Amount	205.1	205.1
Derivative liabilities	83.2	78.0
Carrying Amount [Member] | IPL [Member]
Money market fund investments	21.3
Derivative assets	5.9	10.6
Deferred proceeds (sales of receivables)	32.9	53.7
Long-term debt (including current maturities), Carrying Amount	1,334.1	1,309.0
Cumulative preferred stock of subsidiaries, Carrying Amount	145.1	145.1
Derivative liabilities	38.1	33.6
Carrying Amount [Member] | WPL [Member]
Derivative assets	4.5	5.1
Long-term debt (including current maturities), Carrying Amount	1,082.3	1,082.2
Cumulative preferred stock of subsidiaries, Carrying Amount	60.0	60.0
Derivative liabilities	45.1	44.4
Fair Value [Member]
Money market fund investments	21.3
Derivative assets	10.4	15.7
Deferred proceeds (sales of receivables)	32.9	53.7
Long-term debt (including current maturities), Fair Value	3,317.0	3,325.3
Cumulative preferred stock of subsidiaries, Fair Value	220.6	222.5
Derivative liabilities	83.2	78.0
Fair Value [Member] | IPL [Member]
Money market fund investments	21.3
Derivative assets	5.9	10.6
Deferred proceeds (sales of receivables)	32.9	53.7
Long-term debt (including current maturities), Fair Value	1,582.0	1,560.4
Cumulative preferred stock of subsidiaries, Fair Value	161.4	164.3
Derivative liabilities	38.1	33.6
Fair Value [Member] | WPL [Member]
Derivative assets	4.5	5.1
Long-term debt (including current maturities), Fair Value	1,409.0	1,439.0
Cumulative preferred stock of subsidiaries, Fair Value	59.2	58.2
Derivative liabilities	$ 45.1	$ 44.4