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Income Taxes - Components (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Current income tax expense:      
State income tax $ 0 $ 3 $ 3
Canadian federal and provincial income tax 51 109 63
Total current income tax expense 51 112 66
Deferred income tax expense/(benefit):      
Canadian federal and provincial income tax (70) (46) 132
Total deferred income tax expense/(benefit) (70) (46) 132
Total income tax expense/(benefit) $ (19) $ 66 $ 198