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Stock-Based Compensation - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2019
USD ($)
Period
shares
Dec. 31, 2018
shares
Employee Stock Purchase Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation, option to be granted at percentage of fair value of common stock 85.00%  
Employee Stock Purchase Plan, offering period 12 months  
Number of purchase periods within each offering period | Period 2  
Aggregate number of shares of common stock reserved for future issuance 700,000  
2008 Equity Incentive Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation, option to be granted at percentage of fair value of common stock 100.00%  
Aggregate number of shares of common stock reserved for future issuance 21,300,000  
Stock-based compensation, award term 10 years  
Performance-based stock awards, outstanding 45,000  
Outstanding options and other stock based awards 17,722,000 17,560,000
Number of shares available for future issuance 1,200,000  
2008 Equity Incentive Plan | Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation, vesting period 4 years  
2008 Equity Incentive Plan | Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of unvested Restricted Stock Units 2,399,000 2,001,000 [1]
2008 Equity Incentive Plan | Performance-based Stock or Cash Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee Stock Purchase Plan, authorized shares for issuance 500,000  
Stock option plan granted on cash award | $ $ 1.0  
2008 Equity Incentive Plan | Minimum | Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation, vesting period 3 years  
2008 Equity Incentive Plan | Maximum | Restricted Stock Units (RSUs)    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation, vesting period 4 years  
[1] Includes shares issuable under performance-based restricted stock unit awards.