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	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash held in money market funds	$ 25,367	$ 25,183
Liabilities, Fair value	110,000	60,000
Indebtedness to Roche [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, Fair value	110,000	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	25,367	25,183
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, Fair value	110,000	60,000
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	110,000	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	25,367	25,183
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	$ 25,367	$ 25,183