Share-Based Compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Outstanding at beginning of period	3,214,398	2,633,589	3,280,874
-Cancelled			(132,500)
-Exercised	(1,545,161)	(589,725)	(679,291)
-Granted	1,400,203	1,170,534	164,506
Outstanding at end of period	3,069,440	3,214,398	2,633,589
Weighted Average Exercise Price
Outstanding at beginning of period	$ 2.68	$ 2.06	$ 1.98
-Cancelled			3.72
-Exercised	2.27	2.43	1.65
-Granted	5.72	3.95	3.28
Outstanding at end of period	$ 4.28	$ 2.68	$ 2.06