Debt (Credit Facilities) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Commercial paper:
Amount outstanding		$ 220.6	$ 159.8
Weighted average remaining maturity (in days)		2 days
Weighted average interest rates		0.60%
Available credit facility capacity	[1]	$ 712.4
Parent Company [Member]
Commercial paper:
Amount outstanding		$ 174.8
Weighted average remaining maturity (in days)		2 days
Weighted average interest rates		0.60%
Available credit facility capacity	[1]	$ 125.2
IPL [Member]
Commercial paper:
Amount outstanding		0.0
Available credit facility capacity	[1]	233.0
WPL [Member]
Commercial paper:
Amount outstanding		$ 45.8	$ 19.9
Weighted average remaining maturity (in days)		1 day
Weighted average interest rates		0.40%
Available credit facility capacity	[1]	$ 354.2

[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, 2016.