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Benefit Plans (Estimated Future And Actual Employer Contributions) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2013 Defined benefit pension plans [Member] | Mar. 31, 2013 Defined benefit pension plans [Member] IPL [Member] | Mar. 31, 2013 Defined benefit pension plans [Member] WPL [Member] | Mar. 31, 2013 Other postretirement benefits plans [Member] | Mar. 31, 2013 Other postretirement benefits plans [Member] IPL [Member] | Mar. 31, 2013 Other postretirement benefits plans [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member] IPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Defined benefit pension plans [Member] WPL [Member] | Dec. 31, 2012 Scenario, Forecast [Member] Other postretirement benefits plans [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 2013 |  |  |  |  |  |  |  |  |  | $ 2.4 | [1] | $ 0.8 | [1] | $ 0.2 | [1] | $ 3.0 | $ 0 | $ 3.0
Actual Through March 31, 2013 | $ 0.9 | [1] | $ 0.3 | [1] | $ 0.1 | [1] | $ 0 | $ 0 | $ 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. In addition, IPL and WPL amounts reflect funding for their non-bargaining employees who are participants in the 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Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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=21915506&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 Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: 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. + Details
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