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Decommissioning and Other Provisions - Decommissioning and Restoration (Details)
$ in Millions, $ in Millions
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2017
USD ($)
Disclosure of detailed information about borrowings [line items]        
Undiscounted cash flows required for future removal of facilities $ 1,000      
Centralia Coal Mine | Surety Bonds        
Disclosure of detailed information about borrowings [line items]        
Bonds issued   $ 139   $ 139
Alberta Mine | Letters Of Credit [Member]        
Disclosure of detailed information about borrowings [line items]        
Bonds issued $ 122   $ 120