Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Cash equivalents:
Total Cash equivalents		$ 267,145,000
Marketable securities:
Total Marketable securities	$ 459,256,000	0
Assets, Fair Value Disclosure	507,948,000
U.S. Government securities
Marketable securities:
Total Marketable securities	78,657,000
Money market funds
Cash equivalents:
Total Cash equivalents	28,192,000	267,145,000
Certificates of deposit
Cash equivalents:
Total Cash equivalents	20,500,000
Marketable securities:
Total Marketable securities	66,256,000
Commercial paper
Marketable securities:
Total Marketable securities	314,343,000
Level 1
Cash equivalents:
Total Cash equivalents		0
Marketable securities:
Assets, Fair Value Disclosure	0
Level 1 | U.S. Government securities
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 1 | Commercial paper
Marketable securities:
Total Marketable securities	0
Level 2
Cash equivalents:
Total Cash equivalents		267,145,000
Marketable securities:
Assets, Fair Value Disclosure	507,948,000
Level 2 | U.S. Government securities
Marketable securities:
Total Marketable securities	78,657,000
Level 2 | Money market funds
Cash equivalents:
Total Cash equivalents	28,192,000	267,145,000
Level 2 | Certificates of deposit
Cash equivalents:
Total Cash equivalents	20,500,000
Marketable securities:
Total Marketable securities	66,256,000
Level 2 | Commercial paper
Marketable securities:
Total Marketable securities	314,343,000
Level 3
Cash equivalents:
Total Cash equivalents		0
Marketable securities:
Assets, Fair Value Disclosure	0
Level 3 | U.S. Government securities
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
Level 3 | Commercial paper
Marketable securities:
Total Marketable securities	$ 0