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Share-based Payments - Summary of non-vested stock award activity (Detail) - Non-vested [member]
12 Months Ended
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Outstanding at January 1 333,183 139,962 199,786
Granted 36,229 291,872 36,291
Vested (62,224) (94,208) (94,704)
Forfeited (3,035) (4,443) (1,411)
Outstanding at December 31 304,153 333,183 139,962