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, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	$ 25,183	$ 135,549
Liabilities, Fair value	60,000
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
Indebtedness to Roche [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, Fair value	60,000
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	25,183	128,146
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
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
Liabilities, Fair value	60,000
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
Significant Other Observable Inputs (Level 2) [Member] | Indebtedness to Roche [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, Fair value	60,000
Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	25,183	56,147
Cash Equivalents [Member] | 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	$ 25,183	$ 56,147