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Leases (Finance Lease) (Details) - WPL [Member] - Sheboygan Falls Energy Facility [Member] - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Leases [Line Items] |  |
Finance lease assets, Property, plant and equipment, net | $ 27 | $ 32
Finance lease liabilities, Other current liabilities | 9 | 9
Finance lease liabilities, Sheboygan Falls Energy Facility | 42 | 51
Finance lease liabilities, Total | $ 51 | $ 60
Finance lease, Remaining lease term | 4 years | 5 years
Finance lease, Discount rate, percent | 11.00% | 11.00%
Finance lease, Depreciation expense | $ 6 | $ 6
Finance lease, Interest expense | 6 | 7
Finance lease, Total expense | $ 12 | $ 13

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