Income Taxes - Components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current income tax expense:
State income tax	$ 2	$ 1	$ 1
Canadian federal and provincial income tax	83	83	70
Total current income tax expense	85	84	71
Deferred income tax expense/(benefit):
Federal income tax	57	75	38
State income tax	(4)	7	3
Canadian federal and provincial income tax	(60)	16	100
Total deferred income tax expense/(benefit)	(7)	98	141
Total income tax expense	$ 78	$ 182	$ 212