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Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets (Detail) - Fair Value, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Assets:    
Assets at fair value $ 121,943 $ 128,094
Level 1 [Member]    
Assets:    
Assets at fair value 58,491 72,644
Level 2 [Member]    
Assets:    
Assets at fair value 63,452 55,450
Money Market Funds [Member]    
Assets:    
Assets at fair value 57,491 68,651
Money Market Funds [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 57,491 68,651
U.S. Treasury Securities [Member]    
Assets:    
Assets at fair value 1,000 3,993
U.S. Treasury Securities [Member] | Level 1 [Member]    
Assets:    
Assets at fair value 1,000 3,993
Corporate Bonds [Member]    
Assets:    
Assets at fair value 29,820 26,827
Corporate Bonds [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 29,820 26,827
Commercial Paper [Member]    
Assets:    
Assets at fair value 29,693 20,420
Commercial Paper [Member] | Level 2 [Member]    
Assets:    
Assets at fair value 29,693 20,420
Asset-Backed Securities [Member]    
Assets:    
Assets at fair value 3,939 8,203
Asset-Backed Securities [Member] | Level 2 [Member]    
Assets:    
Assets at fair value $ 3,939 $ 8,203