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Share-Based Payments - Summary of Change in BSA Outstanding Number (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Balance at beginning of period 147,359 143,694  
Balance at end of period 181,008 147,359 143,694
BSA [Member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Balance at beginning of period 147,359 143,694 196,480
Granted during the period 68,000 93,500 25,000
Forfeited during the period 24,902    
Exercised during the period 9,359 89,835 77,786
Expired during the period 0 0 0
Balance at end of period 181,008 147,359 143,694