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Employee Benefit Plans and Share-Based Payments - Summary of Share Based Compensation Plan (Plan 2016-2019) (Parenthetical) (Detail) - Plan 2016-19 [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Bottom of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Average remaining life of plan 7 months 7 months
Top of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Average remaining life of plan 19 months 31 months