Debt (Credit Facilities) (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Debt [Line Items]
Commercial paper outstanding		$ 514.7	$ 441.2
Commercial paper weighted average interest rates		2.60%
Available credit facility capacity	[1]	$ 370.3
IPL [Member]
Debt [Line Items]
Commercial paper outstanding		0.0	50.4
Available credit facility capacity	[1]	135.0
WPL [Member]
Debt [Line Items]
Commercial paper outstanding		$ 138.4	$ 105.5
Commercial paper weighted average interest rates		2.50%
Available credit facility capacity		$ 161.6

[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, 2019.