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Long-Term Debt - Carrying Value of Long Term Debt Instruments (Detail)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
CAD ($)
Disclosure of detailed information about borrowings [Line Items]      
Long-term debt $ 4,769   $ 4,784
4.70% Senior notes [Member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Jun. 23, 2016    
Maturity date Jun. 23, 2046    
Par value   $ 1,000  
Long-term debt $ 1,355   1,246
5.375% Senior notes [Member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Mar. 04, 2016    
Maturity date Mar. 04, 2046    
Par value   750  
Long-term debt $ 1,010   928
3.527% Senior notes [Member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Dec. 02, 2016    
Maturity date Dec. 02, 2026    
Par value   270  
Long-term debt $ 367   338
4.150% Senior notes [Member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Mar. 04, 2016    
Maturity date Mar. 04, 2026    
Par value   1,000  
Long-term debt $ 1,356   1,246
4.90% Senior notes [Member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Sep. 17, 2010    
Maturity date Sep. 17, 2020    
Par value   500  
Long-term debt $ 681   626
5.505% Medium term notes [member]      
Disclosure of detailed information about borrowings [Line Items]      
Issue date Jun. 26, 2008    
Maturity date Jun. 26, 2018    
Par value   $ 400  
Long-term debt     $ 400