Operations and Significant Accounting Policies- Subsequent Events (Details) - Nemadji Trail Energy Center [Member] $ in Millions	Oct. 01, 2021 USD ($)	Sep. 30, 2021 USD ($) MW
Subsequent Event [Line Items]
Approximate Sales Price	$ 20.0
Estimated Capital Expenditures, Including Past Expenditures		$ 700.0
South Shore Energy
Subsequent Event [Line Items]
Gain on Sale after tax	8.5
Development Costs Sold	$ 8.0
Estimated Capital Expenditures, Including Past Expenditures		140.0
Total Project Costs Incurred to Date		$ 15.0
Natural Gas-fired [Member]
Subsequent Event [Line Items]
Generating Capacity to be Jointly Owned (MW) | MW		600
Natural Gas-fired [Member] | Subsequent Event [Member] | Dairyland Power Cooperative
Subsequent Event [Line Items]
Expected Output Entitlement	50.00%
Natural Gas-fired [Member] | Subsequent Event [Member] | Basin Electric Power Cooperative
Subsequent Event [Line Items]
Expected Output Entitlement	30.00%
Natural Gas-fired [Member] | South Shore Energy | Subsequent Event [Member]
Subsequent Event [Line Items]
Expected Output Entitlement	20.00%