Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions	Jun. 30, 2018 USD ($) kV Miles MW
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	$ 248.8
Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	560.0
Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	710.0
Manitoba Hydro [Member] | Great Northern Transmission Line [Member]
Transmission [Line Items]
Project Costs Recovered from Counterparty	129.2
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	300.0
Minnesota Power [Member] | Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	$ 350.0
Manitoba Hydro PPA [Member] | Manitoba Hydro [Member]
Transmission [Line Items]
Output Being Purchased (MW) | MW	250