Goodwill And Other Intangible Assets (Amortization Expense) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Alliant Energy Corporation [Member]
Amortization expense	$ 3.7	$ 3.9	$ 10.7	$ 13.4
Estimated amortization expense			12.6
IPL [Member]
Amortization expense	3.7	3.6	10.2	10.1
Estimated amortization expense			12.1
WPL [Member]
Amortization expense		0.3	0.5	3.3
Estimated amortization expense			$ 0.5