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Share-Based Payments - Change in Number of Free Shares Outstanding (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 15, 2017
Dec. 05, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes In Number Of Shares Outstanding [line items]          
Granted during the period   8,300 625,200    
Free Share [Member]          
Changes In Number Of Shares Outstanding [line items]          
Balance at beginning of period 1,036,850     1,008,329 641,360
Granted during the period 46,500     310,350 750,500
Forfeited during the period 18,650     24,000 61,833
Exercised during the period 241,844     257,829 321,698
Expired during the period 0     0 0
Balance at end of period 822,856     1,036,850 1,008,329