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Financial Instruments - Summary of Derivative Contracts (Parenthetical) (Detail) - Derivative contract [member]
12 Months Ended
Dec. 31, 2018
Bottom of range [member]  
Disclosure of Derivative Contracts [Line items]  
Risk Variable 25.00%
Top of range [member]  
Disclosure of Derivative Contracts [Line items]  
Risk Variable 50.00%