Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Carrying value	$ 266.6		$ 249.6		$ 238.5
Equity (income) / loss	(41.3)		(39.3)		(38.1)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss	(42.1)		(38.7)		(37.8)
American Transmission Company LLC (ATC) [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%	[1]
Carrying value	257.0	[1]	238.8	[1]
Equity (income) / loss	(41.3)	[1]	(37.8)	[1]	(36.9)	[1]
American Transmission Company LLC (ATC) [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%	[1]
Carrying value	257.0	[1]	238.8	[1]
Equity (income) / loss	(41.3)	[1]	(37.8)	[1]	(36.9)	[1]
Wisconsin River Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	7.3		7.7
Equity (income) / loss	(0.8)		(0.9)		(0.9)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	7.3		7.7
Equity (income) / loss	(0.8)		(0.9)		(0.9)
Other Equity [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	2.3		3.1
Equity (income) / loss	0.8		(0.6)		(0.3)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	266.6		249.6
Equity (income) / loss	(41.3)		(39.3)		(38.1)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	264.3		246.5
Equity (income) / loss	$ (42.1)		$ (38.7)		$ (37.8)

[1] Alliant Energy and WPL have the ability to exercise significant influence over ATC’s financial and operating policies through their participation on ATC’s Board of Directors. Refer to Note 19 for information regarding related party transactions with ATC.