Fair Value Measurements (Recurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Money market fund investments		$ 128.3
Derivative assets	15.7	20.9
Foreign exchange contracts, assets		4.8
Deferred proceeds	53.7	152.9
Derivative liabilities	78.0	67.3
Foreign exchange contracts, liabilities		0.1
IPL [Member]
Money market fund investments		5.6
Derivative assets	10.6	12.9
Foreign exchange contracts, assets		4.8
Deferred proceeds	53.7	152.9
WPL [Member]
Derivative liabilities	44.4	43.3
Foreign exchange contracts, liabilities		0.1
Commodity Contracts [Member]
Derivative assets	15.7	16.1
Derivative liabilities	78.0	67.2
Commodity Contracts [Member] | IPL [Member]
Derivative assets	10.6	8.1
Derivative liabilities	33.6	24.0
Commodity Contracts [Member] | WPL [Member]
Derivative assets	5.1	8.0
Derivative liabilities	44.4	43.2
Level 1 [Member]
Money market fund investments		128.3
Level 1 [Member] | IPL [Member]
Money market fund investments		5.6
Level 1 [Member] | WPL [Member]
Derivative liabilities
Foreign exchange contracts, liabilities
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member]
Derivative assets
Derivative liabilities
Level 2 [Member]
Derivative assets	3.4	10.0
Derivative liabilities	64.8	63.9
Level 2 [Member] | IPL [Member]
Derivative assets	1.3	3.6
Level 2 [Member] | WPL [Member]
Derivative liabilities	36.2	40.1
Level 2 [Member] | Commodity Contracts [Member]
Derivative assets	3.4	10.0
Derivative liabilities	64.8	63.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member]
Derivative assets	1.3	3.6
Derivative liabilities	28.6	23.8
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member]
Derivative assets	2.1	6.4
Derivative liabilities	36.2	40.1
Level 3 [Member]
Derivative assets	12.3	10.9
Foreign exchange contracts, assets		4.8
Deferred proceeds	53.7	152.9
Derivative liabilities	13.2	3.4
Foreign exchange contracts, liabilities		0.1
Level 3 [Member] | IPL [Member]
Derivative assets	9.3	9.3
Foreign exchange contracts, assets		4.8
Deferred proceeds	53.7	152.9
Level 3 [Member] | WPL [Member]
Derivative liabilities	8.2	3.2
Foreign exchange contracts, liabilities		0.1
Level 3 [Member] | Commodity Contracts [Member]
Derivative assets	12.3	6.1
Derivative liabilities	13.2	3.3
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member]
Derivative assets	9.3	4.5
Derivative liabilities	5.0	0.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member]
Derivative assets	3.0	1.6
Derivative liabilities	$ 8.2	$ 3.1