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Income Taxes - Components (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Current income tax expense:      
State income tax $ 1 $ 2 $ 1
Canadian federal and provincial income tax 27 83 83
Total current income tax expense 28 85 84
Deferred income tax expense/(benefit):      
Canadian federal and provincial income tax 16 (60) 16
Total deferred income tax expense/(benefit) 16 (60) 16
Total income tax expense $ 44 $ 25 $ 100