Stockholders' Equity - Fair Value of Employee Stock Options Using Weighted Average Assumptions (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Dividend yield	0.70%	1.30%	1.40%
Volatility	39.10%	34.50%	28.60%
Risk-free interest rate	2.20%	1.20%	1.80%
Weighted average expected option term (in years)	4 years 7 months 6 days	5 years 6 months	5 years 7 months 6 days
Weighted average fair value per share of options granted	$ 10.37	$ 4.04	$ 2.80
Aggregate intrinsic value of options exercised	$ 13,086,167	$ 2,391,154	$ 6,534,800