Leases (Future Minimum Operating Lease Payments, Excluding Contingent Rentals) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years
Component of Operating Other Cost and Expense [Line Items]
Underlying lease, remaining terms, years	4
Synthetic Leases [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	43	[1]
2013	2	[1]
2014	4	[1]
2015	1	[1]
Total	50	[1]
Residual value of assets	45
Synthetic Leases [Member] | WPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	1	[1]
2013	2	[1]
2014	4	[1]
2015	1	[1]
Total	8	[1]
Residual value of assets	4
Operating Leases [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	110
2013	28
2014	8
2015	5
2016	3
Thereafter	20
Total	174
Operating Leases [Member] | IPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	4
2013	4
2014	3
2015	3
2016	2
Thereafter	17
Total	33
Operating Leases [Member] | WPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	64
2013	24
2014	5
2015	1
Thereafter	1
Total	95
Others Leases [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	8
2013	9
2014	4
2015	4
2016	3
Thereafter	20
Total	48
Others Leases [Member] | WPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	4
2013	5
2014	1
Thereafter	1
Total	11
Riverside Energy Center (Riverside) PPA [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	59	[2]
2013	17	[2]
Total	76	[2]
Riverside Energy Center (Riverside) PPA [Member] | WPL [Member]
Component of Operating Other Cost and Expense [Line Items]
2012	59	[2]
2013	17	[2]
Total	76	[2]

[1] The synthetic leases relate to the financing of certain corporate headquarters (Alliant Energy) and utility railcars (Alliant Energy and WPL). The entities that lease these assets to Alliant Energy and WPL do not meet consolidation requirements and are not included on Alliant Energy's or WPL's Consolidated Balance Sheets. Alliant Energy and WPL have guaranteed the residual value of the related assets, which total $45 million and $4 million, respectively, in the aggregate. The guarantees extend through the maturity of each respective underlying lease with remaining terms up to four years. Residual value guarantee amounts have been included in the future minimum operating lease payments. Alliant Energy currently plans to exercise its option under the corporate headquarters lease and purchase the building at the expiration of the lease term in April 2012.

[2] In November 2011, WPL filed a Certificate of Authority with the PSCW for the purchase of Riverside in the fourth quarter of 2012. A decision from the PSCW is expected in April 2012. If Riverside is purchased in the fourth quarter of 2012, capacity payments scheduled for 2013 will not occur. Refer to Note 20 for additional information on the Riverside PPA.