Variable Interest Entities (VIEs)	3 Months Ended
Variable Interest Entities (VIEs)	Mar. 31, 2012
Variable Interest Entities (VIEs)	(15) VARIABLE INTEREST ENTITIES (VIEs) After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (a subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if WPL is the primary beneficiary. This PPA is currently accounted for as an operating lease. The counterparty for the Riverside PPA sells a portion of its generating capacity to WPL and can sell its energy output to WPL. Alliant Energy's and WPL's maximum exposure to loss from this PPA is undeterminable due to the inability to obtain the necessary information to complete such evaluation. Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $6.3 million and $6.4 million for the three months ended March 31, 2012 and 2011, respectively. In April 2012, the PSCW approved WPL's Certificate of Authority (CA) application to acquire Riverside for approximately $393 million by the end of 2012. WPL's purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the demand of its customers. WPL currently plans to complete the acquisition in December 2012.
IPL [Member]
Variable Interest Entities (VIEs)	(15) VARIABLE INTEREST ENTITIES (VIEs) After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (a subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if WPL is the primary beneficiary. This PPA is currently accounted for as an operating lease. The counterparty for the Riverside PPA sells a portion of its generating capacity to WPL and can sell its energy output to WPL. Alliant Energy's and WPL's maximum exposure to loss from this PPA is undeterminable due to the inability to obtain the necessary information to complete such evaluation. Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $6.3 million and $6.4 million for the three months ended March 31, 2012 and 2011, respectively. In April 2012, the PSCW approved WPL's Certificate of Authority (CA) application to acquire Riverside for approximately $393 million by the end of 2012. WPL's purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the demand of its customers. WPL currently plans to complete the acquisition in December 2012.
WPL [Member]
Variable Interest Entities (VIEs)	(15) VARIABLE INTEREST ENTITIES (VIEs) After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (a subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if WPL is the primary beneficiary. This PPA is currently accounted for as an operating lease. The counterparty for the Riverside PPA sells a portion of its generating capacity to WPL and can sell its energy output to WPL. Alliant Energy's and WPL's maximum exposure to loss from this PPA is undeterminable due to the inability to obtain the necessary information to complete such evaluation. Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $6.3 million and $6.4 million for the three months ended March 31, 2012 and 2011, respectively. In April 2012, the PSCW approved WPL's Certificate of Authority (CA) application to acquire Riverside for approximately $393 million by the end of 2012. WPL's purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the demand of its customers. WPL currently plans to complete the acquisition in December 2012.