Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets
Total financial assets	$ 849,585	$ 674,408
Level 1
Financial assets
Total financial assets	495,115	88,683
Level 2
Financial assets
Total financial assets	354,470	585,725
Money Market Funds
Financial assets
Total financial assets	495,115	88,683
Money Market Funds | Level 1
Financial assets
Total financial assets	495,115	88,683
Commercial Paper
Financial assets
Total financial assets		121,867
Commercial Paper | Level 2
Financial assets
Total financial assets		121,867
U.S. Government Debt and Agency Securities
Financial assets
Total financial assets	340,836	377,243
U.S. Government Debt and Agency Securities | Level 2
Financial assets
Total financial assets	340,836	377,243
Corporate Bonds
Financial assets
Total financial assets	13,634	86,615
Corporate Bonds | Level 2
Financial assets
Total financial assets	$ 13,634	$ 86,615