Segmented Information (Capital Expenditures) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Payments To Aquire Property Plant and Equipment	$ 1,796	$ 1,132	$ 2,232
Operating Segments [Member] | Canadian Operations [Member]
Segment Reporting Information [Line Items]
Payments To Aquire Property Plant and Equipment	426	256	380
Operating Segments [Member] | USA Operations [Member]
Segment Reporting Information [Line Items]
Payments To Aquire Property Plant and Equipment	1,358	873	1,847
Operating Segments [Member] | Market Optimization [Member]
Segment Reporting Information [Line Items]
Payments To Aquire Property Plant and Equipment	1	1	1
Corporate, Non-segment [Member]
Segment Reporting Information [Line Items]
Payments To Aquire Property Plant and Equipment	$ 11	$ 2	$ 4