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Stock-Based Compensation - Additional Information (Detail)
12 Months Ended
Jan. 01, 2019
shares
Dec. 31, 2018
USD ($)
Milestone
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
Milestone
$ / shares
shares
Dec. 31, 2015
USD ($)
Milestone
shares
Sep. 20, 2018
shares
Jul. 02, 2014
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share based compensation granted under plan vest period   4 years          
Share based compensation, vest period   1 year          
Share based compensation, term of plan   10 years          
Stock-based compensation expense | $   $ 101,963,000 $ 35,534,000 $ 23,020,000      
Weighted average grant date fair value per share | $ / shares   $ 109.92 $ 41.94 $ 24.97      
Share option, weighted-average forfeiture rates   10.70% 13.30% 9.60%      
Total number of options outstanding   232,244          
Non-employees [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share based compensation, term of plan   10 years          
2011 Stock Option Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Options granted   0          
Total number of options outstanding   9,945,522          
2014 Stock Option Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Milestones achieved | Milestone   1   1 1    
Total number of options outstanding   9,945,522          
2014 Stock Option Plan [Member] | Consultant [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Stock-based compensation expense | $       $ 0      
2014 Stock Option Plan [Member] | Milestone One [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percentage of performance based grants that were achieved         35.00%    
2014 Stock Option Plan [Member] | Milestone Two [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percentage of performance based grants that were achieved       50.00% 30.00%    
2014 Stock Option Plan [Member] | Milestone Three [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percentage of performance based grants that were achieved   33.00%   50.00% 35.00%    
2014 Stock Option Plan [Member] | Subsequent Event [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Common stock shares annual increase added to plan 1,875,530            
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%          
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  
Shares of common stock initially authorized for issuance under this plan           1,200,000  
Common stock available for issuance under stock option plan   2,332,055          
2014 Employee Stock Purchase Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Total number of shares reserved for issuance             282,000
Shares of common stock initially authorized for issuance under this plan             282,000
Number of shares issued under the plan   49,756          
Accrued expenses | $   $ 500,000          
Total number of options outstanding   9,945,522          
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   25.00%          
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 $ 0        
Stock-based compensation expense | $   $ 651,000 $ 617,000        
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   71,400          
Performance Restricted Stock Units [Member] | Regulatory Milestones [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock units granted   33,600          
Performance Restricted Stock Units [Member] | Commercial Milestones [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Restricted stock units granted   37,800          
Performance Shares [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Options granted   524,003 449,208 74,039 497,100    
Total unrecognized stock-based compensation expense | $   $ 50,100,000          
Number of shares outstanding and unvested stock options   772,347          
Performance Milestone [Member] | 2014 Stock Option Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Stock-based compensation expense | $   $ 4,400,000   $ 5,000,000 $ 4,800,000    
Performance-Based Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Stock-based compensation expense | $   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          
Stock Options [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Options granted   3,103,494          
Stock-based compensation expense | $   $ 100,342,000 $ 34,525,000 22,820,000      
Total unrecognized stock-based compensation expense | $   $ 282,600,000          
Weighted average period of unrecognized compensation costs   2 years 10 months 17 days          
Intrinsic value of options exercised | $   $ 98,300,000 $ 77,700,000 $ 4,600,000      
Total number of options outstanding   7,530,767 5,586,593