Summary of Significant Accounting Policies - Schedule of Financial Assets That Have Been Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial assets
Total	$ 289,060	$ 148,673
Current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	143,079	66,253
Current [Member] | Commercial Paper [Member]
Financial assets
Investments	43,978	6,720
Current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments	19,124	1,999
Non-current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	2,001	26,916
Non-current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments		2,480
Money Market Funds [Member]
Financial assets
Cash equivalents	76,881	41,805
Certificates of Deposit [Member] | Current [Member]
Financial assets
Investments	3,997	2,500
Level 1 [Member]
Financial assets
Total	76,881	41,805
Level 1 [Member] | Money Market Funds [Member]
Financial assets
Cash equivalents	76,881	41,805
Level 2 [Member]
Financial assets
Total	212,179	106,868
Level 2 [Member] | Current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	143,079	66,253
Level 2 [Member] | Current [Member] | Commercial Paper [Member]
Financial assets
Investments	43,978	6,720
Level 2 [Member] | Current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments	19,124	1,999
Level 2 [Member] | Non-current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	2,001	26,916
Level 2 [Member] | Non-current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments		2,480
Level 2 [Member] | Certificates of Deposit [Member] | Current [Member]
Financial assets
Investments	$ 3,997	$ 2,500