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Benefit Plans (Estimated Future Funding) (Details) - Forecast [Member] $ in Millions | Dec. 31, 2020 USD ($)
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Defined Benefit Pension Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | $ 59.8 | [1]
Defined Benefit Pension Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | 19.4 | [1]
Defined Benefit Pension Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | 21.8 | [1]
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | 8.1 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | 1.8 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Estimated funding for calendar year 2020 | $ 6.0 |
[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 Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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