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Share-based Payments - Summary of non-vested stock award activity (Detail) - Non-vested [member]
12 Months Ended
Dec. 31, 2020
shares
Dec. 31, 2019
shares
Dec. 31, 2018
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Non-vested at January 1 211,205 271,904 304,153
Granted 28,201 28,947 43,355
Vested (83,409) (80,170) (72,045)
Forfeited (2,076) (9,476) (3,559)
Non-vested at December 31 153,921 211,205 271,904