Stockholders' Equity - Weighted Average Fair Values of Employee Stock Options for Option Grants Issued Valuation Assumptions Using Black-Scholes Option Model (Detail) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Equity [Abstract] | |||
| Dividend yield | 1.40% | 1.80% | 1.80% |
| Volatility | 28.60% | 45.20% | 57.80% |
| Risk-free interest rate | 1.80% | 2.00% | 1.50% |
| Weighted average expected option term (in years) | 5 years 7 months 6 days | 5 years 6 months | 5 years 6 months |
| Weighted average fair value per share of options granted | $ 2.80 | $ 2.81 | $ 3.01 |
| Aggregate intrinsic value of options exercised | $ 6,534,800 | $ 819,478 | $ 1,009,726 |