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Benefit Plans (Estimated Future And Actual Employer Contributions) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended |
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Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Alliant Energy [Member] | Sep. 30, 2011 Pension Plans, Defined Benefit [Member] Alliant Energy [Member] | Sep. 30, 2012 Pension Plans, Defined Benefit [Member] Alliant Energy [Member] | Sep. 30, 2011 Pension Plans, Defined Benefit [Member] Alliant Energy [Member] | Sep. 30, 2012 Qualified Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Qualified Defined Benefit Pension Plans [Member] IPL [Member] | Sep. 30, 2012 Qualified Defined Benefit Pension Plans [Member] WPL [Member] | Sep. 30, 2012 Non-qualified defined benefit pension plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] IPL [Member] | Sep. 30, 2012 Directly assigned defined benefit pension plans [Member] WPL [Member] | Sep. 30, 2012 Other postretirement benefits plans [Member] Alliant Energy [Member] | Sep. 30, 2011 Other postretirement benefits plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Other postretirement benefits plans [Member] Alliant Energy [Member] | Sep. 30, 2011 Other postretirement benefits plans [Member] Alliant Energy [Member] | Sep. 30, 2012 Other postretirement benefits plans [Member] IPL [Member] | Sep. 30, 2012 Other postretirement benefits plans [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Pension Plans, Defined Benefit [Member] Alliant Energy [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Qualified Defined Benefit Pension Plans [Member] Alliant Energy [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Qualified Defined Benefit Pension Plans [Member] IPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Qualified Defined Benefit Pension Plans [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Non-qualified defined benefit pension plans [Member] Alliant Energy [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Directly assigned defined benefit pension plans [Member] IPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Directly assigned defined benefit pension plans [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member] Alliant Energy [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member] IPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [Member] WPL [Member]
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated for Calendar Year 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 16.3 | [1] | $ 0.8 | [2] | $ 0.2 | [2] | $ 4.4 | $ 2.2 | $ 2.0
Actual Through September 30, 2012 |  |  |  |  | 0 | 0 | 0 | 3.1 | [1] | 0.7 | [2] | 0.1 | [2] |  |  | 4.4 |  | 2.2 | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement loss | $ 0 | $ 1.1 | $ 0 | $ 1.1 |  |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 0 |  |  | $ 5.0 |  |  |  |  |  |  |  |  |  |  |  |
[1] | Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans. Estimated amounts for calendar year 2012 include amounts paid to a retired executive in the fourth quarter of 2012, which are expected to result in a settlement loss of $5 million in the fourth quarter of 2012.
[2] | Amounts directly assigned to IPL and WPL for non-bargaining employees who are participants in Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.

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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -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 (b)(3) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + Details
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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -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 (g) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + 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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