Fair Value Measurements - Summary of Company's Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	$ 135,549	$ 217,687
Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	56,147	103,141
US Government Agency Securities and Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	79,402	114,546
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	128,146	149,695
Quoted Prices in Active Markets (Level 1) [Member] | Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	56,147	94,741
Quoted Prices in Active Markets (Level 1) [Member] | US Government Agency Securities and Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	71,999	54,954
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	7,403	67,992
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds		8,400
Significant Other Observable Inputs (Level 2) [Member] | US Government Agency Securities and Treasuries [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	$ 7,403	$ 59,592