Variable Interest Entities (VIEs)	9 Months Ended
Variable Interest Entities (VIEs)	Sep. 30, 2012
Variable Interest Entity [Line Items]
Variable Interest Entities (VIEs)	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 $29.7 million and $57.7 million for the three and nine months ended September 30, 2012, and $28.5 million and $55.4 million for the three and nine months ended September 30, 2011, respectively. In April 2012, the PSCW approved WPL’s Certificate of Authority (CA) application to acquire Riverside for approximately $393 million. In June 2012, FERC approved WPL’s application to acquire Riverside. In August 2012, the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (HSR Act), required for WPL to acquire Riverside expired. WPL’s purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the long-term energy needs of its customers. WPL currently plans to complete the acquisition in December 2012.
IPL [Member]
Variable Interest Entity [Line Items]
Variable Interest Entities (VIEs)	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 $29.7 million and $57.7 million for the three and nine months ended September 30, 2012, and $28.5 million and $55.4 million for the three and nine months ended September 30, 2011, respectively. In April 2012, the PSCW approved WPL’s Certificate of Authority (CA) application to acquire Riverside for approximately $393 million. In June 2012, FERC approved WPL’s application to acquire Riverside. In August 2012, the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (HSR Act), required for WPL to acquire Riverside expired. WPL’s purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the long-term energy needs of its customers. WPL currently plans to complete the acquisition in December 2012.
WPL [Member]
Variable Interest Entity [Line Items]
Variable Interest Entities (VIEs)	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 $29.7 million and $57.7 million for the three and nine months ended September 30, 2012, and $28.5 million and $55.4 million for the three and nine months ended September 30, 2011, respectively. In April 2012, the PSCW approved WPL’s Certificate of Authority (CA) application to acquire Riverside for approximately $393 million. In June 2012, FERC approved WPL’s application to acquire Riverside. In August 2012, the waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (HSR Act), required for WPL to acquire Riverside expired. WPL’s purchase of Riverside would replace the 490 MW of electricity output currently obtained from the Riverside PPA to meet the long-term energy needs of its customers. WPL currently plans to complete the acquisition in December 2012.