Income Taxes - Difference Between the Company's Reported Provision for Income Taxes and the U.S. Federal Statutory Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal tax at statutory rate	35.00%	35.00%	35.00%
State and local taxes - net of federal benefit	1.50%	2.90%	3.40%
Credits and deductions related to research activities	(1.20%)	(1.30%)	(1.30%)
Foreign rate differential benefit	(2.90%)	(2.30%)	(2.00%)
Excess tax benefit from stock-based compensation	(11.60%)
Tax Cuts and Jobs Act provisional tax charge	5.80%
Other, net		(0.20%)
Provision for income taxes	26.60%	34.10%	35.10%