Fair Value Measurements - Company's Available-for-Sale Securities by Level within Fair Value Hierarchy (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 429,640	$ 149,974
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	22,409	78,330
U.S. government-related debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	5,003	27,003
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value	1,471	11,971
Corporate equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value		10,518
Money market fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	400,757	22,152
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	400,757	32,670
Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Level 1 | U.S. government-related debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Level 1 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value	0	0
Level 1 | Corporate equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value		10,518
Level 1 | Money market fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	400,757	22,152
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	28,883	117,304
Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	22,409	78,330
Level 2 | U.S. government-related debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	5,003	27,003
Level 2 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value	1,471	11,971
Level 2 | Corporate equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value		0
Level 2 | Money market fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Level 3 | U.S. government-related debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Level 3 | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value	0	0
Level 3 | Corporate equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, at Fair Value		0
Level 3 | Money market fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0	$ 0