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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Debentures (Details) - CAD ($)
$ in Millions
 12 Months Ended
Aug. 02, 2018
Jul. 20, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]          
Notional amount     $ 3,234 $ 3,664  
Borrowings     3,267 3,707  
Alberta off-coal agreement, annual cash payments, gross   $ 40 40 40 $ 0
Alberta off-coal agreement, annual cash payments, net   $ 37      
Debentures          
Disclosure of detailed information about borrowings [line items]          
Notional amount     651 1,051  
Redemption of unsecured debt $ 425        
Redemption of prepayment premium recognized in net interest expense 19        
Accrued and unpaid interest $ 6        
Interest rate 6.40% 6.40%      
Borrowings $ 400   $ 647 $ 1,046  
Debentures | Minimum          
Disclosure of detailed information about borrowings [line items]          
Interest rate       5.00%  
Debentures | Maximum          
Disclosure of detailed information about borrowings [line items]          
Interest rate       7.30%