Derivative Instruments (Fair Value Of Financial Instruments) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Current derivative assets	$ 22.7	$ 19.1
Current derivative liabilities	41.3	55.3
IPL [Member] | Foreign Contracts [Member]
Current derivative assets		4.8
WPL [Member] | Foreign Contracts [Member]
Current derivative liabilities		0.1
Foreign Contracts [Member]
Current derivative assets	0.4	4.8
Current derivative liabilities		0.1
IPL [Member] | Commodity Contracts [Member]
Current derivative assets	15.5	7.8
Non-current derivative assets	1.2	0.3
Current derivative liabilities	17.3	23.0
Non-current derivative liabilities	3.4	1.0
WPL [Member] | Commodity Contracts [Member]
Current derivative assets	6.8	6.5
Non-current derivative assets	1.2	1.5
Current derivative liabilities	24.0	32.2
Non-current derivative liabilities	7.7	11.0
Commodity Contracts [Member]
Current derivative assets	22.3	14.3
Non-current derivative assets	2.4	1.8
Current derivative liabilities	41.3	55.2
Non-current derivative liabilities	11.1	12.0
IPL [Member]
Current derivative assets	15.5	12.6
Current derivative liabilities	17.3	23.0
WPL [Member]
Current derivative assets	6.8	6.5
Current derivative liabilities	$ 24.0	$ 32.3