Utility Rate Cases	9 Months Ended
Utility Rate Cases	Sep. 30, 2012
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 May 2012, WPL filed a retail base rate filing based on a forward-looking test period that includes 2013 and 2014. The filing requested approval for 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 filing also requested authority to maintain customer base rates for WPL’s retail electric customers at their current levels through the end of 2014. Recovery of the costs for the planned acquisition of Riverside, the SCR project at Edgewater Unit 5 and the scrubber and baghouse projects at Columbia Units 1 and 2 are included in the request. The recovery of the costs for these capital projects are offset by decreases in rate base resulting from increased net deferred tax liabilities, the impact of changes in the amortizations of regulatory assets and regulatory liabilities, and the reduction of capacity payments. In July 2012, WPL received an order from the PSCW authorizing WPL to implement its retail base rate filing as requested. Refer to Note 1(b) for details of increases to “Regulatory assets” on Alliant Energy’s and WPL’s Condensed Consolidated Balance Sheets and regulatory-related credits to “Utility - Other operation and maintenance” in Alliant Energy’s and WPL’s Condensed Consolidated Statements of Income during the nine months ended September 30, 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed clean air compliance plan projects. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the Iowa Utilities Board (IUB) to increase annual rates for its Iowa retail gas customers by $15 million, or approximately 6%. The request was based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital investments since IPL’s last Iowa retail gas rate case filed in 2005. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL is proposing to reduce customer bills utilizing a 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, without regulatory review and subject to refund pending determination of final rates from the request. During the three and nine months ended September 30, 2012, Alliant Energy and IPL recorded $2 million and $2 million, respectively, in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In August 2012, IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition filed a unanimous settlement proposal with the IUB addressing all issues among these parties related to this rate case. The unanimous settlement proposal includes an increase in annual rates for IPL’s Iowa retail gas customers of $11 million and utilization of IPL’s proposed tax benefit rider. The unanimous settlement proposal and the gas tax benefit rider for this rate case are subject to approval by the IUB. The IUB established a December 2012 hearing date to address the issues in this rate case and is expected to issue its decision by April 2013. IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers by $15 million, or approximately 22%. The request was based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to Note 1(b) for discussion of changes to regulatory assets and regulatory liabilities during the nine months ended September 30, 2011 based on the MPUC’s decisions to provide IPL’s retail electric customers in Minnesota additional refunds from the gain on the sale of electric transmission assets in 2007 and to provide IPL recovery of $2 million of previously incurred costs for Sutherland #4. Refer to Note 1(c) for discussion of an impairment recognized during the nine months ended September 30, 2011 based on the MPUC’s decision regarding the recovery of IPL’s Whispering Willow - East wind project costs. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In June 2012, WPL filed a request with the PSCW to decrease annual rates for WPL’s retail electric customers by $25 million, or approximately 2%, 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. In October 2012, WPL received an oral decision from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, related to expected changes in retail fuel-related costs. The 2013 fuel-related costs approved by the PSCW were based on forecasted energy market prices for 2013, which were updated in October 2012. WPL currently anticipates the 2013 fuel-related costs will be monitored using an annual bandwidth of plus or minus 2%. The rate change granted from this request is scheduled to be effective on January 1, 2013. WPL’s Retail Fuel-related Rate Case (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million related to expected changes in retail fuel-related costs for 2012. The December 2011 order also required WPL to defer direct CSAPR compliance costs that are not included in the fuel monitoring level and set a zero percent tolerance band for the CSAPR-related deferral. The 2012 fuel-related costs, excluding deferred CSAPR compliance costs, will be monitored using an annual bandwidth of plus or minus 2%. The retail electric rate increase granted from this request was effective January 1, 2012. Retail fuel-related costs incurred by WPL for the period from January 2012 through September 2012 were lower than retail fuel-related costs used to determine rates. WPL currently projects that its retail fuel-related costs for the 2012 calendar year will remain lower than the approved fuel monitoring level by more than the 2% bandwidth resulting in future refunds anticipated to be paid to WPL’s retail electric customers. As of September 30, 2012, Alliant Energy and WPL recorded $5 million in “Regulatory liabilities” on their Condensed Consolidated Balance Sheets for refunds anticipated to be paid to WPL’s retail electric customers.
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 May 2012, WPL filed a retail base rate filing based on a forward-looking test period that includes 2013 and 2014. The filing requested approval for 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 filing also requested authority to maintain customer base rates for WPL’s retail electric customers at their current levels through the end of 2014. Recovery of the costs for the planned acquisition of Riverside, the SCR project at Edgewater Unit 5 and the scrubber and baghouse projects at Columbia Units 1 and 2 are included in the request. The recovery of the costs for these capital projects are offset by decreases in rate base resulting from increased net deferred tax liabilities, the impact of changes in the amortizations of regulatory assets and regulatory liabilities, and the reduction of capacity payments. In July 2012, WPL received an order from the PSCW authorizing WPL to implement its retail base rate filing as requested. Refer to Note 1(b) for details of increases to “Regulatory assets” on Alliant Energy’s and WPL’s Condensed Consolidated Balance Sheets and regulatory-related credits to “Utility - Other operation and maintenance” in Alliant Energy’s and WPL’s Condensed Consolidated Statements of Income during the nine months ended September 30, 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed clean air compliance plan projects. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the Iowa Utilities Board (IUB) to increase annual rates for its Iowa retail gas customers by $15 million, or approximately 6%. The request was based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital investments since IPL’s last Iowa retail gas rate case filed in 2005. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL is proposing to reduce customer bills utilizing a 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, without regulatory review and subject to refund pending determination of final rates from the request. During the three and nine months ended September 30, 2012, Alliant Energy and IPL recorded $2 million and $2 million, respectively, in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In August 2012, IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition filed a unanimous settlement proposal with the IUB addressing all issues among these parties related to this rate case. The unanimous settlement proposal includes an increase in annual rates for IPL’s Iowa retail gas customers of $11 million and utilization of IPL’s proposed tax benefit rider. The unanimous settlement proposal and the gas tax benefit rider for this rate case are subject to approval by the IUB. The IUB established a December 2012 hearing date to address the issues in this rate case and is expected to issue its decision by April 2013. IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers by $15 million, or approximately 22%. The request was based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to Note 1(b) for discussion of changes to regulatory assets and regulatory liabilities during the nine months ended September 30, 2011 based on the MPUC’s decisions to provide IPL’s retail electric customers in Minnesota additional refunds from the gain on the sale of electric transmission assets in 2007 and to provide IPL recovery of $2 million of previously incurred costs for Sutherland #4. Refer to Note 1(c) for discussion of an impairment recognized during the nine months ended September 30, 2011 based on the MPUC’s decision regarding the recovery of IPL’s Whispering Willow - East wind project costs. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In June 2012, WPL filed a request with the PSCW to decrease annual rates for WPL’s retail electric customers by $25 million, or approximately 2%, 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. In October 2012, WPL received an oral decision from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, related to expected changes in retail fuel-related costs. The 2013 fuel-related costs approved by the PSCW were based on forecasted energy market prices for 2013, which were updated in October 2012. WPL currently anticipates the 2013 fuel-related costs will be monitored using an annual bandwidth of plus or minus 2%. The rate change granted from this request is scheduled to be effective on January 1, 2013. WPL’s Retail Fuel-related Rate Case (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million related to expected changes in retail fuel-related costs for 2012. The December 2011 order also required WPL to defer direct CSAPR compliance costs that are not included in the fuel monitoring level and set a zero percent tolerance band for the CSAPR-related deferral. The 2012 fuel-related costs, excluding deferred CSAPR compliance costs, will be monitored using an annual bandwidth of plus or minus 2%. The retail electric rate increase granted from this request was effective January 1, 2012. Retail fuel-related costs incurred by WPL for the period from January 2012 through September 2012 were lower than retail fuel-related costs used to determine rates. WPL currently projects that its retail fuel-related costs for the 2012 calendar year will remain lower than the approved fuel monitoring level by more than the 2% bandwidth resulting in future refunds anticipated to be paid to WPL’s retail electric customers. As of September 30, 2012, Alliant Energy and WPL recorded $5 million in “Regulatory liabilities” on their Condensed Consolidated Balance Sheets for refunds anticipated to be paid to WPL’s retail electric customers.
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 May 2012, WPL filed a retail base rate filing based on a forward-looking test period that includes 2013 and 2014. The filing requested approval for 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 filing also requested authority to maintain customer base rates for WPL’s retail electric customers at their current levels through the end of 2014. Recovery of the costs for the planned acquisition of Riverside, the SCR project at Edgewater Unit 5 and the scrubber and baghouse projects at Columbia Units 1 and 2 are included in the request. The recovery of the costs for these capital projects are offset by decreases in rate base resulting from increased net deferred tax liabilities, the impact of changes in the amortizations of regulatory assets and regulatory liabilities, and the reduction of capacity payments. In July 2012, WPL received an order from the PSCW authorizing WPL to implement its retail base rate filing as requested. Refer to Note 1(b) for details of increases to “Regulatory assets” on Alliant Energy’s and WPL’s Condensed Consolidated Balance Sheets and regulatory-related credits to “Utility - Other operation and maintenance” in Alliant Energy’s and WPL’s Condensed Consolidated Statements of Income during the nine months ended September 30, 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed clean air compliance plan projects. IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the Iowa Utilities Board (IUB) to increase annual rates for its Iowa retail gas customers by $15 million, or approximately 6%. The request was based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital investments since IPL’s last Iowa retail gas rate case filed in 2005. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL is proposing to reduce customer bills utilizing a 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, without regulatory review and subject to refund pending determination of final rates from the request. During the three and nine months ended September 30, 2012, Alliant Energy and IPL recorded $2 million and $2 million, respectively, in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In August 2012, IPL, the Iowa Office of Consumer Advocate (OCA) and the Iowa Consumers Coalition filed a unanimous settlement proposal with the IUB addressing all issues among these parties related to this rate case. The unanimous settlement proposal includes an increase in annual rates for IPL’s Iowa retail gas customers of $11 million and utilization of IPL’s proposed tax benefit rider. The unanimous settlement proposal and the gas tax benefit rider for this rate case are subject to approval by the IUB. The IUB established a December 2012 hearing date to address the issues in this rate case and is expected to issue its decision by April 2013. IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers by $15 million, or approximately 22%. The request was based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to Note 1(b) for discussion of changes to regulatory assets and regulatory liabilities during the nine months ended September 30, 2011 based on the MPUC’s decisions to provide IPL’s retail electric customers in Minnesota additional refunds from the gain on the sale of electric transmission assets in 2007 and to provide IPL recovery of $2 million of previously incurred costs for Sutherland #4. Refer to Note 1(c) for discussion of an impairment recognized during the nine months ended September 30, 2011 based on the MPUC’s decision regarding the recovery of IPL’s Whispering Willow - East wind project costs. WPL’s Retail Fuel-related Rate Case (2013 Test Year) - In June 2012, WPL filed a request with the PSCW to decrease annual rates for WPL’s retail electric customers by $25 million, or approximately 2%, 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. In October 2012, WPL received an oral decision from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, related to expected changes in retail fuel-related costs. The 2013 fuel-related costs approved by the PSCW were based on forecasted energy market prices for 2013, which were updated in October 2012. WPL currently anticipates the 2013 fuel-related costs will be monitored using an annual bandwidth of plus or minus 2%. The rate change granted from this request is scheduled to be effective on January 1, 2013. WPL’s Retail Fuel-related Rate Case (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million related to expected changes in retail fuel-related costs for 2012. The December 2011 order also required WPL to defer direct CSAPR compliance costs that are not included in the fuel monitoring level and set a zero percent tolerance band for the CSAPR-related deferral. The 2012 fuel-related costs, excluding deferred CSAPR compliance costs, will be monitored using an annual bandwidth of plus or minus 2%. The retail electric rate increase granted from this request was effective January 1, 2012. Retail fuel-related costs incurred by WPL for the period from January 2012 through September 2012 were lower than retail fuel-related costs used to determine rates. WPL currently projects that its retail fuel-related costs for the 2012 calendar year will remain lower than the approved fuel monitoring level by more than the 2% bandwidth resulting in future refunds anticipated to be paid to WPL’s retail electric customers. As of September 30, 2012, Alliant Energy and WPL recorded $5 million in “Regulatory liabilities” on their Condensed Consolidated Balance Sheets for refunds anticipated to be paid to WPL’s retail electric customers.