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Asset and Goodwill Impairment Testing - Summary of Key Assumptions Used in Our Asset Impairment Reversal and Goodwill Impairment Analyses (Detail)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of information for cash-generating units [line items]    
Discount rate 6.00% 5.90%
Long-term foreign exchange rate 1 U.S. to 1.25 Canadian dollars 1 U.S. to 1.25 Canadian dollars
Inflation rate 2.00% 2.00%
Steelmaking Coal [member]    
Disclosure of information for cash-generating units [line items]    
Key assumptions used in asset impairment, asset impairment reversal and goodwill impairment Current price used in initial year, decreased to a long-term price in 2023 of US$150 per tonne Current price used in initial year, decreased to a long-term price in 2022 of US$140 per tonne
Copper [member]    
Disclosure of information for cash-generating units [line items]    
Key assumptions used in asset impairment, asset impairment reversal and goodwill impairment Current price used in initial year, increased to a long-term price in 2023 of US$3.00 per pound Current price used in initial year, decreased to a long-term price in 2022 of US$3.00 per pound