XML 48 R57.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Regulatory Matters (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  | 1 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  | 12 Months Ended |  |  |  |  | 8 Months Ended |  | 12 Months Ended |  | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 Commodity cost recovery [Member] | Dec. 31, 2012 Commodity cost recovery [Member] | Dec. 31, 2013 IPL's electric transmission assets sale [Member] | Dec. 31, 2012 IPL's electric transmission assets sale [Member] | Dec. 31, 2013 Cost of removal obligations [Member] | Dec. 31, 2012 Cost of removal obligations [Member] | Dec. 31, 2013 IPL's tax benefit riders [Member] | Dec. 31, 2012 IPL's tax benefit riders [Member] | Dec. 31, 2013 Other [Member] | Dec. 31, 2012 Other [Member] | Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2013 WPL [Member] | Dec. 31, 2012 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2013 WPL [Member] Commodity cost recovery [Member] | Dec. 31, 2012 WPL [Member] Commodity cost recovery [Member] | Dec. 31, 2013 WPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2012 WPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2013 WPL [Member] Cost of removal obligations [Member] | Dec. 31, 2012 WPL [Member] Cost of removal obligations [Member] | Dec. 31, 2013 WPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2012 WPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 WPL [Member] Other [Member] | Dec. 31, 2012 WPL [Member] Other [Member] | Dec. 31, 2013 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member] | Jan. 31, 2013 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Final Order [Member] | Dec. 31, 2013 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Final Order [Member] WPL's Edgewater Unit 5 Purchase [Member] | Dec. 31, 2013 WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member] | Jan. 31, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member] | Sep. 30, 2013 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Jan. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2013 IPL [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2013 IPL [Member] Commodity cost recovery [Member] | Dec. 31, 2012 IPL [Member] Commodity cost recovery [Member] | Dec. 31, 2013 IPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2012 IPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2013 IPL [Member] Cost of removal obligations [Member] | Dec. 31, 2012 IPL [Member] Cost of removal obligations [Member] | Dec. 31, 2013 IPL [Member] Revenue Requirement Adjustment [Member] | Dec. 31, 2013 IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2012 IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 IPL [Member] Other [Member] | Dec. 31, 2012 IPL [Member] Other [Member] | Feb. 28, 2010 IPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2007 IPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2013 IPL [Member] IPL's electric transmission cost recovery [Member] | Dec. 31, 2010 IPL [Member] IPL's electric transmission cost recovery [Member] | Dec. 31, 2011 IPL [Member] Whispering Willow - East Wind Project [Member] Retail Electric Rate [Member] IPL's electric transmission assets sale [Member] | Jan. 31, 2013 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Jun. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Dec. 31, 2013 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] | Jul. 31, 2010 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] Higher Base Rates [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] Renewable Energy Rider [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member] | Dec. 31, 2013 Alliant Energy and IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member] | Nov. 30, 2011 Alliant Energy and IPL [Member] Sutherland 4 [Member] | Dec. 31, 2011 Alliant Energy and IPL [Member] Sutherland 4 [Member] | Dec. 31, 2011 Alliant Energy and IPL [Member] 2009 Test Year Retail Electric IA, Final Order [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2012 Alliant Energy and IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Dec. 31, 2011 Alliant Energy and IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2012 Alliant Energy and WPL [Member] 2013/2014 Test Period Retail Electric And Gas Final Order [Member] | Dec. 31, 2007 SO2 emission allowances [Member] IPL [Member] | Dec. 31, 2012 SO2 emission allowances [Member] Alliant Energy and IPL [Member] | Dec. 31, 2011 SO2 emission allowances [Member] Alliant Energy and IPL [Member] | Dec. 31, 2013 Iowa [Member] IPL [Member] IPL's electric transmission assets sale [Member] | Dec. 31, 2013 Iowa [Member] IPL [Member] IPL's electric transmission assets sale [Member] | Jan. 31, 2014 Subsequent Event [Member] WPL [Member] 2014 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2013 Scenario, Forecast [Member] | Dec. 31, 2013 Scenario, Forecast [Member] WPL [Member] | Dec. 31, 2013 Scenario, Forecast [Member] IPL [Member] | Dec. 31, 2013 Scenario, Forecast [Member] SO2 emission allowances [Member] | Dec. 31, 2013 Scenario, Forecast [Member] SO2 emission allowances [Member] IPL [Member] | Dec. 31, 2013 Electric [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2012 Electric [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2011 Electric [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 Electric [Member] Alliant Energy and IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member] | Dec. 31, 2012 Electric [Member] Alliant Energy and IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member] | Dec. 31, 2011 Electric [Member] Alliant Energy and IPL [Member] IPL's Tax Benefit Rider Decrease In Taxable Income [Member] | Dec. 31, 2014 Electric [Member] Scenario, Forecast [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 Gas [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 Gas [Member] IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [Member] | Dec. 31, 2013 Gas [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 Gas [Member] Scenario, Forecast [Member] IPL's tax benefit riders [Member] | Dec. 31, 2013 Electric And Gas [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Dec. 31, 2014 Revenue Requirement Adjustment [Member] Scenario, Forecast [Member] Alliant Energy and IPL [Member] IPL's tax benefit riders [Member]
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets not earning a return |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 39 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period for simple average of actual costs incurred during test year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum future commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 34 | 34 | 34 |  |  |  | 2,583 | 552 | 2,028 | 34 | [1] | 34 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in regulatory assets | (140.5) | 178.1 | 413.1 |  |  |  |  |  |  |  |  |  |  |  | (69.1) | 49.1 | 84.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (71.4) | 129.0 | 328.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  | 5.0 |  |  | 30.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amounts not probable of recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electric production fuel and energy purchases | 725.0 | 712.3 | 764.5 |  |  |  |  |  |  |  |  |  |  |  | 342.9 | 367.8 | 381.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 382.1 | 344.5 | 383.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other operation and maintenance | 620.7 | 590.0 | 630.2 |  |  |  |  |  |  |  |  |  |  |  | 258.4 | 240.0 | 255.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 362.3 | 350.0 | 375.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.0 |  |  |  | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expenditure collection period after new rate implementation |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery of direct costs, recovery period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of regulatory asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest % |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit to utility other operation and maintenance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory asset impairment charge |  |  | 9 |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery of direct costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery of direct costs, recovery period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities | 821.5 | 916.1 |  | 7.5 | 17.7 | 21.6 | 32.5 | 418.9 | 408.7 | 265.4 | 355.8 | 40.8 | 46.3 |  | 206.6 | 214.7 |  | 2.0 | 12.5 | 0 | 0 | 141.2 | 140.7 | 0 | 0 | 20.0 | 16.4 |  |  |  |  |  |  |  |  |  | 614.9 | 701.4 |  | 5.5 | 5.2 | 21.6 | 32.5 | 277.7 | 268.0 |  | 265.4 | 355.8 | 20.8 | 29.9 |  | 89.0 | 41.0 |  | 3.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 228.9 |  |  |  |  |  |  |  |  | 36.5 |  |  |
Increase (decrease) in regulatory liabilities | (90.8) | 16.4 | 168.3 |  |  |  |  |  |  |  |  |  |  |  | (8.5) | 28.5 | 12.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (82.3) | (12.1) | 156.3 |  |  |  |  |  |  | (24.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (19.0) |  | 5.0 |  |  |  |  | (8.0) |  |  |  |  |  |  |  |  |  | (79.0) | (83.0) | (61.0) |  |  |  | (85.0) | (11.0) |  |  | (12.0) | (90.0) | (15.0)
Income tax expense (benefit) | 53.9 | 89.4 | 69.2 |  |  |  |  |  |  |  |  |  |  |  | 87.2 | 94.6 | 81.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (37.9) | (19.8) | (3.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (4.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (46.0) | (48.0) | (36.0) | (33.0) | (35.0) | (25.0) |  |  |  | (7.0) |  |  |
Gas revenues from rate increase | 464.8 | 396.3 | 476.7 |  |  |  |  |  |  |  |  |  |  |  | 190.9 | 169.6 | 200.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 273.9 | 226.7 | 276.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (11.0) |  |  |  |  |
Reduction of Iowa retail electric customers' rates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liability amortization period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13 |  |  | 29 |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11 |  |  | 11 |  | 8 | 2 | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  | 19 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 36 |  |  |  |
Period after commencement of rate case proceeding |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas tax benefit rider term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |
Authorized increase (decrease) in interim rates, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9 |  |  | 14 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in interim rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase to electric revenues | 2,689.0 | 2,589.3 | 2,635.8 |  |  |  |  |  |  |  |  |  |  |  | 1,197.2 | 1,218.2 | 1,227.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,491.8 | 1,371.1 | 1,408.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 24.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual bandwidth for fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  | 2.00% |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Over-collection of fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  |  | 17 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Over-collection of fuel-related costs outside of approved range |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized refund to customers, including interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Refer to Note 2 for discussion of $34 million of charges recognized by Alliant Energy and IPL in 2011 for forward contracts to purchase SO2 emission allowances.

X
- Definition Amounts Not Probable Of Recovery + References No definition available. + Details
Name: | lnt_AmountsNotProbableOfRecovery
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Cost incurred directly related to electric production fuel and energy purchases. + References No definition available. + Details
Name: | lnt_CostElectricProductionFuelAndEnergyPurchases
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Credit To Utility Other Operation And Maintenance + References No definition available. + Details
Name: | lnt_CreditToUtilityOtherOperationAndMaintenance
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Overcollection Of Retail Fuel Related Costs + References No definition available. + Details
Name: | lnt_OverCollectionOfRetailFuelRelatedCosts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Public Utilities, Expenditure Collection Period After New Rate Implementation + References No definition available. + Details
Name: | lnt_PublicUtilitiesExpenditureCollectionPeriodAfterNewRateImplementation
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Public Utilities, Gas Tax Benefit-Rider Term + References No definition available. + Details
Name: | lnt_PublicUtilitiesGasTaxBenefitRiderTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Public Utilities, Refund to Customers, Including Interest + References No definition available. + Details
Name: | lnt_PublicUtilitiesRefundToCustomersIncludingInterest
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Public Utilities, Requested Rate Increase (Decrease), Adjustment Period Subsequent to Test Year + References No definition available. + Details
Name: | lnt_PublicUtilitiesRequestedRateIncreaseDecreaseAdjustmentPeriodSubsequentToTestYear
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Public Utilities, Requested Rate Increase (Decrease), Simple Average of Actual Costs Incurred During Test Year, Simple Average Basis + References No definition available. + Details
Name: | lnt_PublicUtilitiesRequestedRateIncreaseDecreaseSimpleAverageOfActualCostsIncurredDuringTestYearSimpleAverageBasis
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Rate of fuel costs capacity + References No definition available. + Details
Name: | lnt_RateOfFuelCostsCapacity
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Recovery of DIrect Costs, Recovery Period + References No definition available. + Details
Name: | lnt_RecoveryOfDirectCostsRecoveryPeriod
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reduction In Electric Customer Rates + References No definition available. + Details
Name: | lnt_ReductionInElectricCustomerRates
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | lnt_RegulatoryMattersLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Regulatory Noncurrent Asset, Amortization Period1 + References No definition available. + Details
Name: | lnt_RegulatoryNoncurrentAssetAmortizationPeriod1
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Regulatory Noncurrent Liability, Amortization Period1 + References No definition available. + Details
Name: | lnt_RegulatoryNoncurrentLiabilityAmortizationPeriod1
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_AmortizationOfRegulatoryAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Discloses the amount of any reduction to the carrying amounts of regulated assets that result from impairment (for example, rate actions of a regulator). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6066304&loc=d3e44026-110379 + Details
Name: | us-gaap_AmountOfImpairmentToCarryingAmountOfRegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Current regulatory liabilities generally represent obligations to make refunds to customers for various reasons including overpayment. + References No definition available. + Details
Name: | us-gaap_CustomerRefundLiabilityCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(b)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_ElectricDomesticRegulatedRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of [natural] gas. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(b)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_GasDomesticRegulatedRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
Name: | us-gaap_IncomeTaxExpenseBenefit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in the value of other assets that are created when regulatory agencies permits public utilities to defer costs to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInOtherRegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
Name: | us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The minimum amount the entity agreed to spend under the long-term purchase commitment. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesInterimRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Return of or reimbursements received in relation to direct costs and expenses previously paid or incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_RecoveryOfDirectCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6591318&loc=d3e44360-110383 + Details
Name: | us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(b)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration