Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Carrying value	$ 296.6		$ 281.4		$ 266.6
Equity (income) / loss	(40.4)		(43.7)		(41.3)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss	(42.8)		(43.7)		(42.1)
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%	[1]
Carrying value	286.5	[1]	272.1	[1]
Equity (income) / loss	(41.9)	[1]	(42.7)	[1]	(41.3)	[1]
ATC [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	16.00%	[1]
Carrying value	286.5	[1]	272.1	[1]
Equity (income) / loss	(41.9)	[1]	(42.7)	[1]	(41.3)	[1]
Wisconsin River Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	7.8		7.0
Equity (income) / loss	(0.9)		(1.0)		(0.8)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Carrying value	7.8		7.0
Equity (income) / loss	(0.9)		(1.0)		(0.8)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	2.3		2.3
Equity (income) / loss	2.4		0		0.8
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	296.6		281.4
Equity (income) / loss	(40.4)		(43.7)		(41.3)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	294.3		279.1
Equity (income) / loss	$ (42.8)		$ (43.7)		$ (42.1)

[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 18 for information regarding related party transactions with ATC.