Fair Value Measurements - Summary of Financial Assets Recognized at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 45,799	$ 45,986
Marketable securities:
Marketable securities	226,698	255,203
Total	272,497	301,189
U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	146,713	165,948
Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	53,110	65,420
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	18,892	23,835
Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	7,983
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	45,799	45,986
Marketable securities:
Marketable securities	146,713	165,948
Total	192,512	211,934
Level 1 [Member] | U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	146,713	165,948
Level 2 [Member]
Marketable securities:
Marketable securities	79,985	89,255
Total	79,985	89,255
Level 2 [Member] | Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	53,110	65,420
Level 2 [Member] | Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	18,892	$ 23,835
Level 2 [Member] | Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	$ 7,983