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Financial Instruments - Summary of Foreign Exchange Risk Factor (Parenthetical) (Detail) - Foreign exchange risk [member]
12 Months Ended
Dec. 31, 2018
Bottom of range [member]  
Disclosure of Foreign Exchange Risk Factor [Line items]  
Risk Variable 25.00%
Top of range [member]  
Disclosure of Foreign Exchange Risk Factor [Line items]  
Risk Variable 50.00%