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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
Jun. 30, 2018
Dec. 31, 2017
Assets:    
Assets at fair value $ 209,799 $ 128,094
Liabilities    
Derivative liability 546  
Level 1 [Member]    
Assets:    
Assets at fair value 76,071 72,644
Level 2 [Member]    
Assets:    
Assets at fair value 133,728 55,450
Money Market Funds [Member]    
Assets:    
Assets at fair value 67,611 68,651
Money Market Funds [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 67,611 68,651
U.S. Treasury Securities [Member]    
Assets:    
Assets at fair value 8,460 3,993
U.S. Treasury Securities [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 8,460 3,993
Corporate Bonds [Member]    
Assets:    
Assets at fair value 41,421 26,827
Corporate Bonds [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 41,421 26,827
Commercial Paper [Member]    
Assets:    
Assets at fair value 83,184 20,420
Commercial Paper [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 83,184 20,420
Asset-Backed Securities [Member]    
Assets:    
Assets at fair value 9,123 8,203
Asset-Backed Securities [Member] | Level 2 [Member]    
Assets:    
Assets at fair value $ 9,123 $ 8,203