Fair Value Measurements and Investments - Summary of Financial Assets and Liabilities Measured and Recorded at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	$ 40,375	$ 49,926
U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	11,869	14,982
Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	12,942	8,078
Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	12,964	10,314
Asset-backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	2,505	6,467
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	95	10,085
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	11,964	25,067
Level 1 [Member] | U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	11,869	14,982
Level 1 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 [Member] | Asset-backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	95	10,085
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	28,411	24,859
Level 2 [Member] | U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	12,942	8,078
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	12,964	10,314
Level 2 [Member] | Asset-backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	2,505	6,467
Level 2 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member] | U.S. Government Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member] | Asset-backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value	$ 0	$ 0