Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions	Sep. 30, 2020 USD ($) kV Miles MW
Great Northern Transmission Line [Member]
Transmission [Line Items]
Total Project Costs To Date	$ 660.0
Minnesota Power [Member] | Great Northern Transmission Line [Member]
Transmission [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500
Total Project Costs Incurred to Date	$ 310.0
Manitoba Hydro PPA [Member] | Manitoba Hydro [Member]
Transmission [Line Items]
Output Being Purchased (MW) | MW	250