Debt (Credit Facilities) (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Debt [Line Items]
Commercial paper outstanding		$ 390.3	$ 244.1
Commercial paper weighted average interest rates		1.20%
Available credit facility capacity	[1]	$ 569.7
IPL [Member]
Debt [Line Items]
Commercial paper outstanding		$ 4.0	0.0
Commercial paper weighted average interest rates		1.40%
Available credit facility capacity	[1]	$ 256.0
WPL [Member]
Debt [Line Items]
Commercial paper outstanding		$ 224.6	$ 52.3
Commercial paper weighted average interest rates		1.10%
Available credit facility capacity		$ 175.4

[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at September 30, 2017.