Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Commercial paper:
Amount outstanding	$ 243.4		$ 217.5
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.20%
Available credit facility capacity	721.6	[1]
Parent Company [Member]
Commercial paper:
Amount outstanding	83.4
Weighted average remaining maturity (in days)	2 days
Weighted average interest rates	0.30%
Available credit facility capacity	216.6	[1]
IPL [Member]
Commercial paper:
Amount outstanding	0		26.3
Available credit facility capacity	265.0	[1]
WPL [Member]
Commercial paper:
Amount outstanding	160.0		86.6
Weighted average remaining maturity (in days)	3 days
Weighted average interest rates	0.20%
Available credit facility capacity	$ 240.0	[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 March 31, 2013. Refer to Note 8(b) for further discussion of $35 million of commercial paper outstanding at March 31, 2013 classified as long-term debt on Alliant Energy’s and IPL’s Condensed Consolidated Balance Sheets.