Fair Value Measurements - Summary of Financial Assets Recognized at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 35,538	$ 45,986
Marketable securities:
Marketable securities	257,350	255,203
Total	292,888	301,189
U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	176,626	165,948
Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	53,823	65,420
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	18,906	23,835
Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	7,995
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	35,538	45,986
Marketable securities:
Marketable securities	176,626	165,948
Total	212,164	211,934
Level 1 [Member] | U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	176,626	165,948
Level 2 [Member]
Marketable securities:
Marketable securities	80,724	89,255
Total	80,724	89,255
Level 2 [Member] | Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	53,823	65,420
Level 2 [Member] | Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	18,906	$ 23,835
Level 2 [Member] | Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	$ 7,995