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Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets and Liabilities (Detail) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Assets:    
Assets at fair value $ 183,619 $ 128,094
Liabilities:    
Derivative liability 602  
Level 1 [Member]    
Assets:    
Assets at fair value 74,192 72,644
Level 2 [Member]    
Assets:    
Assets at fair value 109,427 55,450
Liabilities:    
Derivative liability 46  
Level 3 [Member]    
Liabilities:    
Derivative liability 556  
Derivative Liability Exit Fee [Member]    
Liabilities:    
Derivative liability 556  
Derivative Liability Exit Fee [Member] | Level 3 [Member]    
Liabilities:    
Derivative liability 556  
Foreign Exchange Contract [Member]    
Liabilities:    
Derivative liability 46  
Foreign Exchange Contract [Member] | Level 2 [Member]    
Liabilities:    
Derivative liability 46  
Money Market Funds [Member]    
Assets:    
Assets at fair value 66,235 68,651
Money Market Funds [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 66,235 68,651
U.S. Treasury Securities [Member]    
Assets:    
Assets at fair value 7,957 3,993
U.S. Treasury Securities [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 7,957 3,993
Corporate Bonds [Member]    
Assets:    
Assets at fair value 36,439 26,827
Corporate Bonds [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 36,439 26,827
Commercial Paper [Member]    
Assets:    
Assets at fair value 65,105 20,420
Commercial Paper [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 65,105 20,420
Asset-Backed Securities [Member]    
Assets:    
Assets at fair value 7,883 8,203
Asset-Backed Securities [Member] | Level 2 [Member]    
Assets:    
Assets at fair value $ 7,883 $ 8,203