Available-for-Sale Securities and Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments:
Total investments	$ 177,692	$ 196,421
Agency Securities
Investments:
Total investments	21,256	17,352
Corporate Debt Securities
Investments:
Total investments	44,153	54,474
Commercial Paper
Investments:
Total investments	11,033	5,965
U.S. Government Securities
Investments:
Total investments	101,250	118,630
Fair Value Measurements Recurring
Cash and cash equivalents
Total cash and cash equivalents	72,564	107,511
Investments:
Total investments	177,692	196,421
Fair Value Measurements Recurring | Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	72,564	107,511
Fair Value Measurements Recurring | Agency Securities
Investments:
Total investments	21,256	17,352
Fair Value Measurements Recurring | Corporate Debt Securities
Investments:
Total investments	44,153	54,474
Fair Value Measurements Recurring | Commercial Paper
Investments:
Total investments	11,033	5,965
Fair Value Measurements Recurring | U.S. Government Securities
Investments:
Total investments	101,250	118,630
Level 1 | Fair Value Measurements Recurring
Cash and cash equivalents
Total cash and cash equivalents	72,564	107,511
Level 1 | Fair Value Measurements Recurring | Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	72,564	107,511
Level 2 | Fair Value Measurements Recurring
Investments:
Total investments	177,692	196,421
Level 2 | Fair Value Measurements Recurring | Agency Securities
Investments:
Total investments	21,256	17,352
Level 2 | Fair Value Measurements Recurring | Corporate Debt Securities
Investments:
Total investments	44,153	54,474
Level 2 | Fair Value Measurements Recurring | Commercial Paper
Investments:
Total investments	11,033	5,965
Level 2 | Fair Value Measurements Recurring | U.S. Government Securities
Investments:
Total investments	$ 101,250	$ 118,630