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Related Parties - Changes of Number of Shares Granted (Detail) - shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Related parties details 3 [abstract]    
Share granted, beginning 1,183,398 1,500,072
Shares granted on March 4, 2017   100,000
Forfeiture of granted shares due to termination of executive employment (216,666) (143,333)
Shares vested and transferred (116,668) (273,341)
Share granted, ending 850,064 1,183,398