Fair Value of Financial Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Money Market Mutual Funds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Money market mutual funds (included in cash and cash equivalents)	$ 76,949	$ 20,698
Money Market Mutual Funds [Member] | Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Money market mutual funds (included in cash and cash equivalents)	76,949	20,698
Certificates of Deposit [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	3,667	22,046
Certificates of Deposit [Member] | Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	3,667	22,046
Agency Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure		5,913
Agency Bonds [Member] | Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure		5,913
United States Treasury Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	17,627	68,716
United States Treasury Securities [Member] | Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	17,627	68,716
Short-term Investments [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	21,294	96,675
Short-term Investments [Member] | Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	17,627	68,716
Short-term Investments [Member] | Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short-term investments fair value disclosure	$ 3,667	$ 27,959