Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Marketable securities:
Fair Value	$ 364,996	$ 184,123
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	605,236	280,430
Corporate Bonds [Member]
Marketable securities:
Fair Value	352,890	166,292
Municipal Securities [Member]
Marketable securities:
Fair Value	3,028	3,037
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	240,240	96,307
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	7,589	12,804
Certificates of Deposit [Member]
Marketable securities:
Fair Value	1,489	1,990
Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	240,240	96,307
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	364,996	184,123
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	605,236	280,430
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Marketable securities:
Fair Value	352,890	166,292
Significant Other Observable Inputs (Level 2) [Member] | Municipal Securities [Member]
Marketable securities:
Fair Value	3,028	3,037
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	240,240	96,307
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	7,589	12,804
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Marketable securities:
Fair Value	1,489	1,990
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 240,240	$ 96,307