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Benefit Plans (Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2011 Corporate Service Allocation [Member] IPL [Member] | Sep. 30, 2011 Corporate Service Allocation [Member] IPL [Member] | Sep. 30, 2011 Corporate Service Allocation [Member] WPL [Member] | Sep. 30, 2011 Corporate Service Allocation [Member] WPL [Member] | Dec. 31, 2011 Defined Benefit Pension Plans [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] IPL [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] IPL [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] IPL [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] IPL [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] WPL [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] WPL [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] WPL [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] WPL [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member] | Sep. 30, 2012 Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member] | Sep. 30, 2011 Defined Benefit Pension Plans [Member] Alliant Energy Corporation [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] IPL [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] IPL [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] IPL [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] IPL [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] WPL [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] WPL [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] WPL [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] WPL [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member] | Sep. 30, 2012 Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member] | Sep. 30, 2011 Other Postretirement Benefits Costs [Member] Alliant Energy Corporation [Member] | Sep. 30, 2011 Cash Balance Plan [Member] | Sep. 30, 2011 Cash Balance Plan [Member] | Sep. 30, 2011 Cash Balance Plan [Member] IPL [Member] | Sep. 30, 2011 Cash Balance Plan [Member] IPL [Member] | Sep. 30, 2011 Cash Balance Plan [Member] WPL [Member] | Sep. 30, 2011 Cash Balance Plan [Member] WPL [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] IPL [Member] | Sep. 30, 2011 Directly assigned defined benefit pension plans [Member] IPL [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] IPL [Member] | Sep. 30, 2011 Directly assigned defined benefit pension plans [Member] IPL [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] WPL [Member] | Sep. 30, 2011 Directly assigned defined benefit pension plans [Member] WPL [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] WPL [Member] | Sep. 30, 2011 Directly assigned defined benefit pension plans [Member] WPL [Member]
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate additional payments to certain plan participants | $ 0.2 | $ 3.5 | $ 0.3 | $ 2.7 | $ 10.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | [1] | $ 1.3 | [1] | $ 0 | [1] | $ 10.2 | [1] |  |  |  |  |  |  |  |  | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 1.3 | $ 10.2 | $ 0.8 | $ 6.3 | $ 0.4 | $ 3.4 | $ 0 | [1] | $ 0.6 | [1] | $ 0 | [1] | $ 2.8 | [1] | $ 0 | [1] | $ 0.1 | [1] | $ 0 | [1] | $ 0.7 | [1]
Benefits costs associated with Corporate Services employees |  |  |  |  |  | 0.4 | [2] | 1.3 | [2] | 1.4 | [2] | 5.4 | [2] | 0.4 | [2] | 0.9 | [2] | 1.0 | [2] | 3.9 | [2] |  |  |  |  |  |  |  |  | 0 | 0 | 0.1 | 0.4 | 0.1 | (0.1) | 0.1 | 0.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 1.1 |  | $ 0 |  | $ 1.1 |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  | $ 0 |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in the second quarter of 2011, resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance Plan in the second half of 2011. Alliant Energy recognized $1.3 million and $10.2 million of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. IPL recognized $0.8 million ($0.6 million directly assigned and $0.2 million allocated by Corporate Services) and $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. WPL recognized $0.4 million ($0.1 million directly assigned and $0.3 million allocated by Corporate Services) and $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.
[2] | For the three and nine months ended September 30, 2011, additional qualified pension benefits costs resulting from the amendment to the Cash Balance Plan in the second quarter of 2011 allocated to IPL were $0.2 million and $3.5 million, and to WPL were $0.3 million and $2.7 million, respectively.

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- Definition Pension and other postretirement benefits plans costs allocated from Corporate Services. + References No definition available. + Details
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- Definition The amount of net gain (loss) recognized in net periodic benefit cost as a result of an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include (a) making lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and (b) purchasing nonparticipating annuity contracts to cover vested benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Number 88 -Name Statement of Financial Accounting Standard (FAS) -Paragraph 3 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph h -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 15 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6412315&loc=d3e8001-114927 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 106 -Paragraph 518 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 264 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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