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Stock-Based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Jan. 01, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Sep. 20, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Total number of options outstanding   10,001,048           10,001,048      
Common stock available for issuance under stock option plan   2,422,437           2,422,437      
Stock-based compensation expense   $ 25,478,000     $ 9,731,000     $ 70,323,000 $ 25,289,000    
Weighted average grant date fair value per share               $ 113.12 $ 40.43    
2011 Stock Option Plan [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Options granted               0      
2016 Stock Option Plan [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Total number of shares reserved for issuance                     1,200,000
2014 Stock Option Plan [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Common stock shares annual increase added to plan 1,680,117                    
2014 Stock Option Plan [Member] | Maximum [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Percentage of increase on outstanding shares of Common stock                   4.00%  
Restricted Stock Units [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Restricted stock units granted               71,400 32,500    
Share based compensation granted under plan vest period               2 years      
Fair value of restricted stock units vested               $ 2,600,000      
Restricted stock units vested   0           14,550 0    
Stock-based compensation expense   $ 164,000     $ 176,000     $ 487,000 $ 444,000    
Restricted Stock Units [Member] | Restricted Stock Units Vest One Year Anniversary [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Restricted stock units cliff vesting percentage               50.00%      
Restricted stock units vesting period month and year               2018-02      
Restricted Stock Units [Member] | Restricted Stock Units Vest Two Year Anniversary [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Restricted stock units cliff vesting percentage               50.00%      
Restricted stock units vesting period month and year               2019-02      
Performance Restricted Stock Units [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Restricted stock units granted     37,800 33,600              
Performance Shares [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Options granted   0 200,250 323,753 0 0 449,208        
Total unrecognized stock-based compensation expense   $ 58,400,000           $ 58,400,000      
Number of shares outstanding and unvested stock options   1,098,536           1,098,536      
Performance-Based Stock Options [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Stock-based compensation expense               $ 0 0    
Performance-Based Stock Options [Member] | Consultant [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Stock-based compensation expense     $ 6,900,000                
Stock Options [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Options granted               2,775,594      
Total number of options outstanding   7,494,711           7,494,711   5,586,593  
Stock-based compensation expense   $ 25,015,000     $ 9,414,000     $ 69,023,000 24,565,000    
Total unrecognized stock-based compensation expense   $ 291,800,000           $ 291,800,000      
Weighted average period of unrecognized compensation costs               2 years 11 months 8 days      
Intrinsic value of options exercised               $ 93,000,000 $ 14,100,000