Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Commercial paper:
Amount outstanding	$ 162.8		$ 102.8
Weighted average remaining maturity (in days)	4 days
Weighted average interest rates	0.40%
Available credit facility capacity	787.2	[1]
Parent Company [Member]
Commercial paper:
Amount outstanding	111.8
Weighted average remaining maturity (in days)	5 days
Weighted average interest rates	0.40%
Available credit facility capacity	188.2	[1]
IPL [Member]
Commercial paper:
Amount outstanding	15.4		7.1
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.40%
Available credit facility capacity	234.6	[1]
WPL [Member]
Commercial paper:
Amount outstanding	35.6		25.7
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.30%
Available credit facility capacity	$ 364.4	[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 June 30, 2012. Refer to Note 7(b) for further discussion of $50 million of commercial paper outstanding at June 30, 2012 classified as long-term debt.