Available-for-Sale Securities and Fair Value Measurements - Summary of Aggregate Market Value, Cost Basis, and Gross Unrealized Gains and Losses of Available for Sale Investments by Security Type (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	$ 177,456	$ 196,529
Gross Unrealized Gains	236	14
Gross Unrealized Losses		(122)
Total investments	177,692	196,421
Agency Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	21,243	17,361
Gross Unrealized Gains	13
Gross Unrealized Losses		(9)
Total investments	21,256	17,352
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	44,051	54,536
Gross Unrealized Gains	102
Gross Unrealized Losses		(62)
Total investments	44,153	54,474
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	11,033	5,965
Total investments	11,033	5,965
U.S. Government Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	101,129	118,667
Gross Unrealized Gains	121	14
Gross Unrealized Losses		(51)
Total investments	$ 101,250	$ 118,630