Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 459,145,000
Gross Unrealized Gains	139,000
Gross Unrealized Losses	(28,000)
Estimated Fair Value	459,256,000	$ 0
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	314,292,000
Gross Unrealized Gains	74,000
Gross Unrealized Losses	(23,000)
Estimated Fair Value	314,343,000
U.S. Government securities
Marketable Securities [Line Items]
Amortized Cost	78,612,000
Gross Unrealized Gains	48,000
Gross Unrealized Losses	(3,000)
Estimated Fair Value	78,657,000
Certificates of deposit
Marketable Securities [Line Items]
Amortized Cost	66,241,000
Gross Unrealized Gains	17,000
Gross Unrealized Losses	(2,000)
Estimated Fair Value	$ 66,256,000