Fair Value Measurements - Schedule of Fair Value of Financial Instruments (Detail) - Recurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	$ 470,167	$ 282,686
Total liabilities measured at fair value	3,101	2,952
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	339,635	136,258
U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	25,243	48,355
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	61,295	77,082
Overnight repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value		15,001
U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	43,994	5,990
Derivative liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	3,101	2,952
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	364,878	184,613
Total liabilities measured at fair value	0	0
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	339,635	136,258
Level 1 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	25,243	48,355
Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 | Overnight repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value		0
Level 1 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 | Derivative liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	105,289	98,073
Total liabilities measured at fair value	0	0
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 2 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	61,295	77,082
Level 2 | Overnight repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value		15,001
Level 2 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	43,994	5,990
Level 2 | Derivative liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Total liabilities measured at fair value	3,101	2,952
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | Overnight repurchase agreements
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value		0
Level 3 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | Derivative liability
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	$ 3,101	$ 2,952