Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Marketable securities:
Total Marketable securities	$ 461,848	$ 459,256
Total	610,907	507,948
U.S. Government securities
Cash equivalents:
Total Cash equivalents	21,900
Marketable securities:
Total Marketable securities	135,293	78,657
Other debt securities
Marketable securities:
Total Marketable securities	9,999
Commercial paper
Cash equivalents:
Total Cash equivalents	16,495
Marketable securities:
Total Marketable securities	270,636	314,343
Money market funds
Cash equivalents:
Total Cash equivalents	93,156	28,192
Certificates of deposit
Cash equivalents:
Total Cash equivalents	17,508	20,500
Marketable securities:
Total Marketable securities	45,920	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 | Other debt securities
Marketable securities:
Total Marketable securities	0
Level 1 | Commercial paper
Cash equivalents:
Total Cash equivalents	0
Marketable securities:
Total Marketable securities	0	0
Level 1 | Money market funds
Cash equivalents:
Total Cash equivalents	0	0
Level 1 | Certificates of deposit
Cash equivalents:
Total Cash equivalents	0	0
Marketable securities:
Total Marketable securities	0	0
Level 2
Marketable securities:
Total	610,907	507,948
Level 2 | U.S. Government securities
Cash equivalents:
Total Cash equivalents	21,900
Marketable securities:
Total Marketable securities	135,293	78,657
Level 2 | Other debt securities
Marketable securities:
Total Marketable securities	9,999
Level 2 | Commercial paper
Cash equivalents:
Total Cash equivalents	16,495
Marketable securities:
Total Marketable securities	270,636	314,343
Level 2 | Money market funds
Cash equivalents:
Total Cash equivalents	93,156	28,192
Level 2 | Certificates of deposit
Cash equivalents:
Total Cash equivalents	17,508	20,500
Marketable securities:
Total Marketable securities	45,920	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 | Other debt securities
Marketable securities:
Total Marketable securities	0
Level 3 | Commercial paper
Cash equivalents:
Total Cash equivalents	0
Marketable securities:
Total Marketable securities	0	0
Level 3 | Money market funds
Cash equivalents:
Total Cash equivalents	0	0
Level 3 | Certificates of deposit
Cash equivalents:
Total Cash equivalents	0	0
Marketable securities:
Total Marketable securities	$ 0	$ 0