Stockholders' Equity - Weighted Average Assumptions Used for the Fair Value of the Option Component of the Purchase Plan Shares Estimated at Grant Date Using Black-Scholes Option Pricing Model (Detail) - Employee Stock Purchase Plan - $ / shares |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
Expected term (in years) | 1 year 3 months 18 days | 1 year 2 months 12 days | 1 year 2 months 12 days |
Expected volatility | 35.20% | 26.80% | 30.50% |
Risk-free interest rate | 2.20% | 1.00% | 0.70% |
Expected dividends | 0.00% | 0.00% | 0.00% |
Weighted average fair value at grant date (usd per share) | $ 94.71 | $ 31.36 | $ 22.23 |