Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions	Sep. 30, 2019 USD ($) kV Miles MW
Great Northern Transmission Line [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	$ 700.0
Total Project Costs Incurred to Date	581.6
Manitoba Hydro [Member] | Great Northern Transmission Line [Member]
Transmission [Line Items]
Project Costs Recovered from Counterparty	$ 310.4
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 Cost in the U.S.	$ 325.0
Manitoba Hydro PPA [Member] | Manitoba Hydro [Member]
Transmission [Line Items]
Output Being Purchased (MW) | MW	250