Fair Value of Financial Instruments - Schedule of Financial Assets That Have Been Measured at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets
Total	$ 225,034	$ 289,060
Corporate Debt Securities [Member]
Financial assets
Cash equivalents		6,823
Commercial Paper [Member]
Financial assets
Cash equivalents	12,985	7,738
US Government and Agency Securities [Member]
Financial assets
Cash equivalents	1,499
Current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	112,105	143,079
Current [Member] | Commercial Paper [Member]
Financial assets
Investments	45,607	43,978
Current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments	19,207	19,124
Non-current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments		2,001
Money Market Funds [Member]
Financial assets
Cash equivalents	29,632	62,320
Certificates of Deposit [Member] | Current [Member]
Financial assets
Investments	3,999	3,997
Level 1 [Member]
Financial assets
Total	29,632	62,320
Level 1 [Member] | Money Market Funds [Member]
Financial assets
Cash equivalents	29,632	62,320
Level 2 [Member]
Financial assets
Total	195,402	226,740
Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets
Cash equivalents		6,823
Level 2 [Member] | Commercial Paper [Member]
Financial assets
Cash equivalents	12,985	7,738
Level 2 [Member] | US Government and Agency Securities [Member]
Financial assets
Cash equivalents	1,499
Level 2 [Member] | Current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments	112,105	143,079
Level 2 [Member] | Current [Member] | Commercial Paper [Member]
Financial assets
Investments	45,607	43,978
Level 2 [Member] | Current [Member] | US Government and Agency Securities [Member]
Financial assets
Investments	19,207	19,124
Level 2 [Member] | Non-current [Member] | Corporate Debt Securities [Member]
Financial assets
Investments		2,001
Level 2 [Member] | Certificates of Deposit [Member] | Current [Member]
Financial assets
Investments	$ 3,999	$ 3,997