Fair Value Measurements - Fair Value Hierarchy of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Recurring - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	$ 33,120	$ 75,650
Fair value assets and liabilities measured on recurring basis, total	33,120	75,650
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	33,120	75,650
Fair value assets and liabilities measured on recurring basis, total	33,120	75,650
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	0	0
Fair value assets and liabilities measured on recurring basis, total	0	0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	0	0
Fair value assets and liabilities measured on recurring basis, total	$ 0	$ 0