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Debt - Summary of Debt Continuity (Detail)
CAD in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]            
Borrowings at beginning of period CAD 8,343          
Cash flows            
Issuance of debt | $       $ 1,227    
Non-cash changes            
Borrowings at end of balance 6,369   CAD 8,343      
Debt Continuity [member]            
Disclosure of detailed information about borrowings [line items]            
Borrowings at beginning of period 8,343 $ 6,213 9,634 6,961    
Cash flows            
Issuance of debt     1,567 1,227    
Scheduled debt repayments (64) (49)        
Debt repurchases (1,831) (1,356) (2,531) (1,960)    
Finance lease payments (c) (34)   (29)   $ (26) $ (22)
Non-cash changes            
Issuance of debt     22 17    
Loss (gain) on debt repurchases (a)(b) 141 105 (49) (37)    
Changes in foreign exchange rates (424)   (308)      
Finance lease liabilities (c) 234 187 28 21    
Finance fees and discount amortization 4 3 9 6    
Borrowings at end of balance CAD 6,369 $ 5,077 CAD 8,343 $ 6,213