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Stockholders' Equity - Summary of Weighted Average Assumptions Used to Estimate Fair Value of Stock Option (Detail) - Stock Options [Member] - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
[1]
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected option term (in years) 6 years 0 years 6 years 6 years
Risk-free interest rate 2.80%   2.70% 2.00%
Expected volatility 35.00%   35.00% 40.00%
Weighted average exercise price $ 31.36   $ 38.34 $ 38.00
Weighted average fair value at grant date $ 12.18   $ 14.83 $ 15.72
[1] No stock options were granted during the three months ended September 30, 2017.