Utility Rate Cases	3 Months Ended
Utility Rate Cases	Mar. 31, 2013
Utility Rate Cases [Line Items]
Utility Rate Cases	UTILITY RATE CASES WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the Public Service Commission of Wisconsin (PSCW) authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013 followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual return on common equity exceeds certain levels during the test period and allows WPL to request a change in retail base rates during the test period if its annual return on common equity falls below a certain level. As of March 31, 2013, WPL did not record any deferred amounts for these provisions. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers. IPL’s request included a proposal to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012. In November 2012, the IUB approved a settlement agreement between IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition related to IPL’s request, resulting in an increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. Refer to Note 4 for additional details on IPL’s gas tax benefit rider. IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million for the year ended December 31, 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. For the three months ended March 31, 2013, Alliant Energy and IPL recognized $5.5 million of the revenue requirement adjustment resulting in an increase to electric revenues on their Condensed Consolidated Statements of Income. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail electric production fuel and energy purchases costs (fuel-related costs) in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.
IPL [Member]
Utility Rate Cases [Line Items]
Utility Rate Cases	UTILITY RATE CASES WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the Public Service Commission of Wisconsin (PSCW) authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013 followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual return on common equity exceeds certain levels during the test period and allows WPL to request a change in retail base rates during the test period if its annual return on common equity falls below a certain level. As of March 31, 2013, WPL did not record any deferred amounts for these provisions. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers. IPL’s request included a proposal to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012. In November 2012, the IUB approved a settlement agreement between IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition related to IPL’s request, resulting in an increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. Refer to Note 4 for additional details on IPL’s gas tax benefit rider. IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million for the year ended December 31, 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. For the three months ended March 31, 2013, Alliant Energy and IPL recognized $5.5 million of the revenue requirement adjustment resulting in an increase to electric revenues on their Condensed Consolidated Statements of Income. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail electric production fuel and energy purchases costs (fuel-related costs) in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.
WPL [Member]
Utility Rate Cases [Line Items]
Utility Rate Cases	UTILITY RATE CASES WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the Public Service Commission of Wisconsin (PSCW) authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013 followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual return on common equity exceeds certain levels during the test period and allows WPL to request a change in retail base rates during the test period if its annual return on common equity falls below a certain level. As of March 31, 2013, WPL did not record any deferred amounts for these provisions. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers. IPL’s request included a proposal to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012. In November 2012, the IUB approved a settlement agreement between IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition related to IPL’s request, resulting in an increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. Refer to Note 4 for additional details on IPL’s gas tax benefit rider. IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million for the year ended December 31, 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. For the three months ended March 31, 2013, Alliant Energy and IPL recognized $5.5 million of the revenue requirement adjustment resulting in an increase to electric revenues on their Condensed Consolidated Statements of Income. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail electric production fuel and energy purchases costs (fuel-related costs) in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.