Debt (Credit Facilities) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Commercial paper:
Amount outstanding		$ 120.9	$ 141.3
Weighted average remaining maturity (in days)		6 days
Weighted average interest rates		0.40%
Available credit facility capacity	[1]	$ 767.9
Commercial paper, long-term		111.2
Parent Company [Member]
Commercial paper:
Amount outstanding		$ 120.9
Weighted average remaining maturity (in days)		6 days
Weighted average interest rates		0.40%
Available credit facility capacity	[1]	$ 179.1
IPL [Member]
Commercial paper:
Amount outstanding		0.0
Available credit facility capacity	[1]	188.8
WPL [Member]
Commercial paper:
Amount outstanding		0.0
Available credit facility capacity	[1]	$ 400.0

[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, 2015. Refer to Note 7(b) for further discussion of $111.2 million of commercial paper outstanding at June 30, 2015 classified as long-term debt on Alliant Energy’s and IPL’s balance sheets.