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Debt - Additional Information (Detail)
CAD in Millions
12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 1,250,000,000
Cost of purchases       1,267,000,000
Pre-tax gain (loss) on notes repurchased | CAD     CAD 27  
After-tax gain (loss) on notes repurchased | CAD     23  
Net proceeds from issuance of notes       1,227,000,000
Notes Purchased [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 1,260,000,000   759,000,000
Cost of purchases   1,360,000,000   693,000,000
Pre-tax gain (loss) on notes repurchased | CAD CAD (216)   76  
After-tax gain (loss) on notes repurchased | CAD CAD (159)   CAD 67  
3.0% notes due March 2019 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 278,000,000   $ 222,000,000
Borrowings, interest rate 3.00% 3.00%   3.00%
4.5% notes due January 2021 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 280,000,000    
Borrowings, interest rate 4.50% 4.50%    
8.0% notes due June 2021 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 650,000,000    
Borrowings, interest rate 8.00% 8.00%    
Loss on write-off of prepayment Option | CAD CAD 75      
Unsecured notes issued       $ 650,000,000
Maturity date June 2021 June 2021    
Effective interest rate       8.49%
4.75% notes due January 2022 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 28,000,000    
Borrowings, interest rate 4.75% 4.75%    
3.75% notes due February 2023 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased   $ 24,000,000   $ 80,000,000
Borrowings, interest rate 3.75% 3.75%   3.75%
6.125% notes due October 2035 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 91,000,000
Borrowings, interest rate       6.125%
6.0% notes due August 2040 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 159,000,000
Borrowings, interest rate       6.00%
6.25% notes due July 2041 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 205,000,000
Borrowings, interest rate       6.25%
5.2% notes due March 2042 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 101,000,000
Borrowings, interest rate       5.20%
5.4% notes due February 2043 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 123,000,000
Borrowings, interest rate       5.40%
8.50% notes due June 2024 [member]        
Disclosure of detailed information about borrowings [line items]        
Borrowings, interest rate 8.50% 8.50%    
Unsecured notes issued       $ 600,000,000
3.15% notes due January 2017 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 266,000,000
Borrowings, interest rate       3.15%
3.85% notes due August 2017 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 284,000,000
Borrowings, interest rate       3.85%
2.5% notes due February 2018 [member]        
Disclosure of detailed information about borrowings [line items]        
Principal amount of notes repurchased       $ 478,000,000
Borrowings, interest rate       2.50%