Variable Interest Entities (VIEs)	12 Months Ended
Variable Interest Entities (VIEs)	Dec. 31, 2011
Variable Interest Entities (VIEs)	(20) VARIABLE INTEREST ENTITIES (VIEs) An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties or its equity investors lack any one of the following three characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity's economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the financial statements of the VIE. After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if Alliant Energy 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. In 2011, 2010 and 2009, Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $62 million, $61 million and $63 million, respectively.
IPL [Member]
Variable Interest Entities (VIEs)	(20) VARIABLE INTEREST ENTITIES (VIEs) An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties or its equity investors lack any one of the following three characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity's economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the financial statements of the VIE. After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if Alliant Energy 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. In 2011, 2010 and 2009, Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $62 million, $61 million and $63 million, respectively.
WPL [Member]
Variable Interest Entities (VIEs)	(20) VARIABLE INTEREST ENTITIES (VIEs) An entity is considered a VIE if its equity investors do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties or its equity investors lack any one of the following three characteristics: (1) power, through voting rights or similar rights, to direct the activities of the entity that most significantly impact the entity's economic performance; (2) the obligation to absorb expected losses of the entity; or (3) the right to receive expected benefits of the entity. The primary beneficiary of a VIE is required to consolidate the financial statements of the VIE. After making an ongoing exhaustive effort, Alliant Energy and WPL concluded they were unable to obtain the information necessary from the counterparty (subsidiary of Calpine Corporation) for the Riverside PPA for Alliant Energy and WPL to determine whether the counterparty is a VIE and if Alliant Energy 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. In 2011, 2010 and 2009, Alliant Energy's (primarily WPL's) costs, excluding fuel costs, related to the Riverside PPA were $62 million, $61 million and $63 million, respectively.