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Income Taxes (Reconciliation of Unrecognized Tax Benefits on Domestic and Foreign Tax Positions) (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Reconciliation for unrecognized tax benefits on the Company's domestic and foreign tax positions      
Gross unrecognized tax benefits at beginning of year $ 74 $ 74 $ 61
Increases for:      
Tax positions related to the current year 5 12 13
Tax positions related to prior years 0 2 2
Decreases for:      
Tax positions related to prior years (17) (13) 0
Settlements 0 (1) (1)
Lapse of the applicable statute of limitations 0 0 (1)
Gross unrecognized tax benefits at end of year 62 74 74
Adjustments to reflect tax treaties and other arrangements (2) (5) (5)
Net unrecognized tax benefits at end of year $ 60 $ 69 $ 69