Income Taxes - Summary of Effective Income Tax Rate Differs from Amount Computed by Applying U.S. Federal Statutory Income Tax Rate to Income Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Expected tax expense	$ 29,762	$ 29,361	$ 26,521
Change in valuation allowance	(21)	22	19
Impact of tax holiday	(4,396)	(4,027)	(2,991)
Foreign tax rate differential	(2,616)	(2,716)	(2,797)
Deferred tax (benefit)/provision	(1,887)	(878)	(803)
Unrecognized tax benefits and interest	(3,905)	495	324
State taxes, net of Federal taxes	339	202	1,327
Non-deductible expenses	825	144	26
Prior year tax expense/(benefit)		0	2,450
US Tax Reform Act impact	29,185	0	0
Excess tax benefit for stock-based compensation	(9,797)	0	0
Other	(1,343)	(452)	135
Income tax expense	$ 36,146	$ 22,151	$ 24,211