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Share-Based Payment Awards - Employee Stock Purchase Plan - Additional Information (Detail) - shares
Jun. 25, 2019
Jun. 30, 2019
Dec. 31, 2018
Class of Stock [Line Items]      
Common stock, shares issued   106,297,792 95,372,236
Employee Stock Purchase Plan [Member]      
Class of Stock [Line Items]      
Shares of common stock reserved for issuance 1,100,000    
Price of common stock purchased under ESPP, percent 85.00%    
Offering period, start date Aug. 01, 2019    
Offering period, end date Jan. 31, 2020    
Common stock, shares issued   0