Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Marketable securities:
Total Marketable securities	$ 476,321	$ 459,256
Total	617,697	507,948
U.S. government securities
Cash equivalents:
Total Cash equivalents	20,600
Marketable securities:
Total Marketable securities	476,321	78,657
Commercial paper
Marketable securities:
Total Marketable securities		314,343
Money market funds
Cash equivalents:
Total Cash equivalents	120,776	28,192
Certificates of deposit
Cash equivalents:
Total Cash equivalents		20,500
Marketable securities:
Total Marketable securities		66,256
Level 1
Marketable securities:
Total	0	0
Level 1 | U.S. government securities
Cash equivalents:
Total Cash equivalents	0
Marketable securities:
Total Marketable securities	0	0
Level 1 | Commercial paper
Marketable securities:
Total Marketable securities		0
Level 1 | Money market funds
Cash equivalents:
Total Cash equivalents	0	0
Level 1 | Certificates of deposit
Cash equivalents:
Total Cash equivalents		0
Marketable securities:
Total Marketable securities		0
Level 2
Marketable securities:
Total	617,697	507,948
Level 2 | U.S. government securities
Cash equivalents:
Total Cash equivalents	20,600
Marketable securities:
Total Marketable securities	476,321	78,657
Level 2 | Commercial paper
Marketable securities:
Total Marketable securities		314,343
Level 2 | Money market funds
Cash equivalents:
Total Cash equivalents	120,776	28,192
Level 2 | Certificates of deposit
Cash equivalents:
Total Cash equivalents		20,500
Marketable securities:
Total Marketable securities		66,256
Level 3
Marketable securities:
Total	0	0
Level 3 | U.S. government securities
Cash equivalents:
Total Cash equivalents	0
Marketable securities:
Total Marketable securities	0	0
Level 3 | Commercial paper
Marketable securities:
Total Marketable securities		0
Level 3 | Money market funds
Cash equivalents:
Total Cash equivalents	$ 0	0
Level 3 | Certificates of deposit
Cash equivalents:
Total Cash equivalents		0
Marketable securities:
Total Marketable securities		$ 0