Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Carrying value		$ 302.9	$ 296.6	$ 281.4
Equity (income) / loss		(33.8)	(40.4)	(43.7)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss		$ (35.1)	(42.8)	(43.7)
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	16.00%
Carrying value	[1]	$ 293.3	286.5
Equity (income) / loss	[1]	$ (34.2)	(41.9)	(42.7)
ATC [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	16.00%
Carrying value	[1]	$ 293.3	286.5
Equity (income) / loss	[1]	$ (34.2)	(41.9)	(42.7)
Wisconsin River Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 8.7	7.8
Equity (income) / loss		$ (0.9)	(0.9)	(1.0)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 8.7	7.8
Equity (income) / loss		(0.9)	(0.9)	(1.0)
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		0.9	2.3
Equity (income) / loss		1.3	2.4	0.0
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		302.9	296.6
Equity (income) / loss		(33.8)	(40.4)	(43.7)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		302.0	294.3
Equity (income) / loss		$ (35.1)	$ (42.8)	$ (43.7)

[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.