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Employee Benefit Plans and Share-Based Payments - Summary of Share Based Compensation Plan (Plan 2014-2017) (Parenthetical) (Detail) - Plan 2014-17 [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Remaining life of plan 7 months 7 months  
Bottom of range [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Remaining life of plan     7 months
Top of range [member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Remaining life of plan     19 months