Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Commercial paper:
Amount outstanding	$ 70.4		$ 102.8
Weighted average remaining maturity (in days)	0 years 0 months 1 day
Weighted average interest rates	0.40%
Available credit facility capacity	879.6	[1]
Parent Company [Member]
Commercial paper:
Amount outstanding	70.4
Weighted average remaining maturity (in days)	0 years 0 months 1 day
Weighted average interest rates	0.40%
Available credit facility capacity	229.6	[1]
IPL [Member]
Commercial paper:
Amount outstanding	0		7.1
Available credit facility capacity	250.0	[1]
WPL [Member]
Commercial paper:
Amount outstanding	0		25.7
Available credit facility capacity	$ 400.0	[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 September 30, 2012. Refer to Note 7(b) for further discussion of $50 million of commercial paper outstanding at September 30, 2012 classified as long-term debt.