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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Senior Notes (Details)
$ in Millions, $ in Millions
 12 Months Ended
Mar. 15, 2018
USD ($)
Mar. 15, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
CAD ($)
Disclosure of detailed information about borrowings [line items]                
Notional amount         $ 3,104 $ 3,234    
Borrowings         $ 3,212 $ 3,267    
Senior notes | Minimum                
Disclosure of detailed information about borrowings [line items]                
Interest rate     4.50%     4.50%    
Senior notes | Maximum                
Disclosure of detailed information about borrowings [line items]                
Interest rate     6.50%     6.50%    
6.65% Senior notes, due May 2018                
Disclosure of detailed information about borrowings [line items]                
Interest rate 6.65%   6.65%     6.65%    
Borrowings $ 500   $ 500          
Redemption of noncurrent borrowings $ 516 $ 617 $ 516 $ 617        
Redemption of prepayment premium recognized in net interest expense   $ 5   5        
Accrued and unpaid interest       $ 14        
4.75% Senior Notes due 2015 | Senior notes                
Disclosure of detailed information about borrowings [line items]                
Interest rate             1.90% 1.90%
Notional amount             $ 400  
Borrowings               $ 434
Non-Recourse Bonds                
Disclosure of detailed information about borrowings [line items]                
Interest rate     4.509%     4.509%