Debt (Credit Facilities) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Debt [Line Items]
Commercial paper outstanding		$ 336.4	$ 320.2
Commercial paper weighted average interest rates		2.30%
Available credit facility capacity	[1]	$ 617.7
IPL [Member]
Debt [Line Items]
Commercial paper outstanding		0.0
Available credit facility capacity	[1]	204.1
WPL [Member]
Debt [Line Items]
Commercial paper outstanding		$ 30.6	$ 25.0
Commercial paper weighted average interest rates		1.80%
Available credit facility capacity		$ 319.4

[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at March 31, 2018.