Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) kV Miles MW
Great Northern Transmission Line [Member]
Transmission [Line Items]
Transmission Line Length (Miles) | Miles	220
Transmission Line Capacity (kV) | kV	500
Great Northern Transmission Line [Member] | Minimum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	$ 560
Great Northern Transmission Line [Member] | Maximum [Member]
Transmission [Line Items]
Total Project Cost in the U.S.	$ 710
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires May 2035) [Member]
Transmission [Line Items]
Output Being Purchased (MW) | MW	250