Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Amount outstanding	$ 57.0		$ 102.8
Remaining maturity, in days	2 days
Weighted average interest rates	0.30%
Available credit facility capacity	918.0	[1]
Commercial paper outstanding classified as long-term debt	25
Parent Company [Member]
Amount outstanding	33.7
Remaining maturity, in days	2 days
Weighted average interest rates	0.40%
Available credit facility capacity	266.3	[1]
IPL [Member]
Amount outstanding			7.1
Remaining maturity, in days	N/A
Available credit facility capacity	275.0	[1]
WPL [Member]
Amount outstanding	23.3		25.7
Remaining maturity, in days	2 days
Weighted average interest rates	0.30%
Available credit facility capacity	$ 376.7	[1]

[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, 2012. Refer to Note 7(b) for further discussion of $25 million of commercial paper outstanding at March 31, 2012 classified as long-term debt.