Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Carrying value		$ 421.3	$ 381.4	$ 326.0
Equity (income) / loss		(54.6)	(44.8)	(39.6)
WPL [Member]
Schedule of Equity Method Investments [Line Items]
Equity (income) / loss		(0.9)	(0.7)	(39.8)
ATC Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value	[1]	293.6	274.2
Equity (income) / loss	[1]	$ (38.1)	(42.4)	(39.1)
Non-utility wind farm in Oklahoma [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 105.1	98.3
Equity (income) / loss		(15.6)	(1.8)	0.0
Other [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		22.6	8.9
Equity (income) / loss		$ (0.9)	(0.6)	(0.5)
ATC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	16.00%
ATC [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		0.00%
Carrying value		$ 0.0	0.0
Equity (income) / loss		$ 0.0	0.0	(39.1)
Wisconsin River Power Company [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest		50.00%
Carrying value		$ 8.1	8.3
Equity (income) / loss		(0.9)	(0.7)	(0.7)
Totals [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		421.3	381.4
Equity (income) / loss		(54.6)	(44.8)	(39.6)
Totals [Member] | WPL [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value		8.1	8.3
Equity (income) / loss		$ (0.9)	$ (0.7)	$ (39.8)
ATC Holdco LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest	[1]	20.00%

[1] As of December 31, 2018, Alliant Energy’s interest in ATC Holdings is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. Alliant Energy currently has the ability to exercise significant influence over ATC’s and ATC Holdco LLC’s financial and operating policies through its participation on ATC’s Board of Directors.