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Fair Value of Financial Instruments - Summary of Composition of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 350,892 $ 90,570
Unrealized Gains 28  
Unrealized Losses (121) (204)
Aggregate Fair Value 350,799 90,366
Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,770 12,446
Unrealized Losses (3) (7)
Aggregate Fair Value 4,767 12,439
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 77,460 67,126
Unrealized Gains 2  
Unrealized Losses (103) (179)
Aggregate Fair Value 77,359 66,947
US Government Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,999 10,998
Unrealized Losses (9) (18)
Aggregate Fair Value 8,990 $ 10,980
US Treasury Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 259,663  
Unrealized Gains 26  
Unrealized Losses (6)  
Aggregate Fair Value $ 259,683