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Stockholders' Equity - Weighted Average Assumptions Stock Options Granted (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity [Abstract]      
Expected life from date of grant (years) 5 years 3 months 4 years 5 months 6 years 5 months
Expected volatility 42.00% 39.80% 35.00%
Expected dividend yield 0.00% 0.00% 0.00%
Risk-free interest rate 1.97% 1.16% 1.81%