Employee Share-Based Compensation Plans - Summary of Stock Option Activity Under the Equity Plans (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares Under Options, Beginning Balance	2,850,330	3,749,783
Shares under options, Granted	177,000	45,000
Shares Under Options, Exercised	(1,178,617)	(883,085)
Shares Under Options, Forfeited or expired	(386,348)	(61,368)
Shares Under Options, Ending Balance	1,462,365	2,850,330	3,749,783
Weighted Average Exercise Price, Beginning Balance	$ 4.36	$ 4.15
Shares Under Options, Vested and expected to vest at December 31, 2017	1,421,378
Weighted Average Exercise Price, Granted	$ 6.94	7.61
Shares Under Options, Exercisable at December 31, 2017	1,285,060
Weighted Average Exercise Price, Exercised	$ 3.52	3.64
Weighted Average Exercise Price, Forfeited or expired	6.25	4.64
Weighted Average Exercise Price, Ending Balance	4.85	$ 4.36	$ 4.15
Weighted Average Exercise Price, Vested and expected to vest at December 31, 2017	4.79
Weighted Average Exercise Price, Exercisable at December 31, 2017	$ 4.55
Weighted Average Remaining Contractual Term, Outstanding	2 years 6 months 3 days	2 years 7 months 17 days	2 years 11 months 4 days
Weighted Average Remaining Contractual Term, Vested and Expected to vest	2 years 5 months 16 days
Weighted Average Remaining Contractual Term, Exercisable	2 years 2 months 27 days
Aggregate Intrinsic Value, Outstanding	$ 2,435	$ 8,961	$ 14,456
Aggregate Intrinsic Value, Vested and expected to vest	2,202
Aggregate Intrinsic Value, Exercisable	$ 2,435