v2.4.0.6
Commitments And Contingencies (Operating Expense Purchase Obligations) (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2012
Long-term Purchase Commitment [Line Items]  
Individual commitments incurred $ 1
Scenario, Forecast [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 1,131
Scenario, Forecast [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 605
Scenario, Forecast [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 355
Scenario, Forecast [Member] | Other [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 22 [1]
Scenario, Forecast [Member] | Other [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 9 [1]
Scenario, Forecast [Member] | Other [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 13 [1]
Scenario, Forecast [Member] | Natural Gas [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 344
Scenario, Forecast [Member] | Natural Gas [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 208
Scenario, Forecast [Member] | Natural Gas [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 136
Scenario, Forecast [Member] | Coal [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 302 [2]
Scenario, Forecast [Member] | Coal [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 79 [2]
Scenario, Forecast [Member] | Coal [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 52 [2]
Scenario, Forecast [Member] | Coal [Member] | Corporate Services [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 171
Scenario, Forecast [Member] | SO2 emission allowances [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 34
Scenario, Forecast [Member] | SO2 emission allowances [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 34
Scenario, Forecast [Member] | SO2 emission allowances [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 0
Scenario, Forecast [Member] | Purchased Power [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 429 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 275 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 154 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 271 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 271 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Duane Arnold Energy Center (DAEC) (IPL) [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 0 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 95 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 0 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Kewaunee Nuclear Power Plant (Kewaunee) WPL [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 95 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | Alliant Energy [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 63 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | IPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments 4 [3]
Scenario, Forecast [Member] | Purchased Power [Member] | Other [Member] | WPL [Member]
  
Long-term Purchase Commitment [Line Items]  
Minimum future commitments $ 59 [3]
[1] Includes individual commitments incurred during the normal course of business that exceeded $1 million at September 30, 2012.
[2] Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments of $171 million that have not been directly assigned to IPL and WPL since the specific needs of each utility were not yet known as of September 30, 2012.
[3] Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. Excludes contracts that are considered operating leases.