Document And Entity Information (USD $) In Billions, except Share data, unless otherwise specified	12 Months Ended			12 Months Ended		12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Jan. 31, 2012 Alliant Energy Corporation [Member]	Dec. 31, 2011 IPL [Member]	Jan. 31, 2012 IPL [Member]	Dec. 31, 2011 WPL [Member]	Jan. 31, 2012 WPL [Member]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Trading Symbol	lnt
Entity Registrant Name	ALLIANT ENERGY CORP			INTERSTATE POWER & LIGHT CO		WISCONSIN POWER & LIGHT CO
Entity Central Index Key	0000352541			0000052485		0000107832
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer			Non-accelerated Filer		Non-accelerated Filer
Entity Common Stock, Shares Outstanding			111,008,651		13,370,788		13,236,601
Entity Well-known Seasoned Issuer	Yes
Entity Public Float		$ 4.5
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No