Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Commercial paper:
Amount outstanding	$ 217.5		$ 102.8
Weighted average interest rates	0.40%		0.30%
Weighted average remaining maturity	11 days		3 days
Available credit facility capacity	732.5	[1]	897.2	[1]
IPL [Member]
Commercial paper:
Amount outstanding	26.3		7.1
Weighted average interest rates	0.40%		0.40%
Weighted average remaining maturity	2 days		3 days
Available credit facility capacity	223.7	[1]	292.9	[1]
WPL [Member]
Commercial paper:
Amount outstanding	86.6		25.7
Weighted average interest rates	0.30%		0.30%
Weighted average remaining maturity	19 days		3 days
Available credit facility capacity	313.4	[1]	374.3	[1]
Long-term debt [Member]
Commercial paper:
Amount outstanding	50.0	[2]	0	[2]
Long-term debt [Member] | IPL [Member]
Commercial paper:
Amount outstanding	$ 50.0	[2]	$ 0	[2]

[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at December 31, 2012. Refer to Note 9(b) for further discussion of $50.0 million of commercial paper outstanding at December 31, 2012 classified as long-term debt on Alliant Energy’s and IPL’s Consolidated Balance Sheets.

[2] As of December 31, 2012, $50.0 million of commercial paper was recorded in “Long-term debt, net” on Alliant Energy’s and IPL’s Consolidated Balance Sheets due to the existence of long-term credit facilities that back-stop this commercial paper balance, along with Alliant Energy’s and IPL’s intent and ability to refinance these balances on a long-term basis. As of December 31, 2012, this commercial paper balance had a remaining maturity of 8 days.