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Benefit Plans (Estimated Future Funding) (Details) - Scenario, Forecast [Member] $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Defined Benefit Pension Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | $ 6.5 | [1]
Defined Benefit Pension Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | 0.7 | [1]
Defined Benefit Pension Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | 0.1 | [1]
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | 8.7 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | 2.0 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2017 | $ 6.4 |
[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 Amount of employer's best estimate 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. + 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=65897934&loc=d3e1928-114920 + Details
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