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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Senior Notes (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
USD ($)
Disclosure of detailed information about borrowings [line items]          
Notional amount $ 3,234     $ 3,664  
Borrowings $ 3,267     $ 3,707  
Senior notes | Minimum          
Disclosure of detailed information about borrowings [line items]          
Interest rate       4.50%  
Senior notes | Maximum          
Disclosure of detailed information about borrowings [line items]          
Interest rate       6.50%  
6.65% Senior notes, due May 2018          
Disclosure of detailed information about borrowings [line items]          
Interest rate 6.65%   6.65%    
Borrowings $ 500   $ 500    
Redemption of noncurrent borrowings 617 $ 516      
Redemption of prepayment premium recognized in net interest expense 5        
Accrued and unpaid interest $ 14        
4.75% Senior Notes due 2015 | Designated as hedging instrument          
Disclosure of detailed information about borrowings [line items]          
Notional amount         $ 480
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%