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Goodwill - Schedule of Reconciliation of Changes in Goodwill (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of reconciliation of changes in goodwill [line items]    
Beginning balance $ 1,087 $ 1,114
Changes in foreign exchange rates 34 (27)
Ending balance 1,121 1,087
Steelmaking Coal Operations [member]    
Disclosure of reconciliation of changes in goodwill [line items]    
Beginning balance 702 702
Ending balance 702 702
Quebrada Blanca [member]    
Disclosure of reconciliation of changes in goodwill [line items]    
Beginning balance 385 412
Changes in foreign exchange rates 34 (27)
Ending balance $ 419 $ 385