Debt (Credit Facilities) (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Debt [Line Items]
Commercial paper outstanding		$ 82.5	$ 320.2
Commercial paper weighted average interest rates		2.20%
Available credit facility capacity	[1]	$ 792.5
IPL [Member]
Debt [Line Items]
Commercial paper outstanding		0.0
Available credit facility capacity	[1]	125.0
WPL [Member]
Debt [Line Items]
Commercial paper outstanding		$ 26.4	$ 25.0
Commercial paper weighted average interest rates		2.00%
Available credit facility capacity		$ 323.6

[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at June 30, 2018.