Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 315,452	$ 457,534
Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	103,008	35,935
Short-term marketable securities	315,452	457,534
Total cash equivalents and short-term marketable securities	418,460	493,469
Fair Value, Measurements, Recurring Basis | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	103,008	35,935
Short-term marketable securities	315,452	457,534
Total cash equivalents and short-term marketable securities	418,460	493,469
Fair Value, Measurements, Recurring Basis | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term marketable securities	0	0
Total cash equivalents and short-term marketable securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Short-term marketable securities	0	0
Total cash equivalents and short-term marketable securities	0	0
Fair Value, Measurements, Recurring Basis | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	103,008	35,935
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	103,008	35,935
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring Basis | U.S. Treasuries
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	315,452	457,534
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	315,452	457,534
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	0	0
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term marketable securities	$ 0	$ 0