Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Amortized Cost	$ 476,329	$ 459,145
Gross Unrealized Gains	75	139
Gross Unrealized Losses	(83)	(28)
Estimated Fair Value	476,321	459,256
U.S. government securities
Marketable Securities [Line Items]
Amortized Cost, due within one year	454,878	78,612
Amortized Cost, due after on year through five years	21,451
Gross Unrealized Gains, due within one year	75	48
Gross Unrealized Gains, due after one year through five years	0
Gross Unrealized Losses, due within one year	(63)	(3)
Gross Unrealized Losses, due after one year through five years	(20)
Estimated Fair Value, due within one year	454,890	78,657
Estimated Fair Value, due after on year through five years	21,431
Estimated Fair Value	$ 476,321	78,657
Commercial paper
Marketable Securities [Line Items]
Amortized Cost, due within one year		314,292
Gross Unrealized Gains, due within one year		74
Gross Unrealized Losses, due within one year		(23)
Estimated Fair Value, due within one year		314,343
Certificates of deposit
Marketable Securities [Line Items]
Amortized Cost, due within one year		66,241
Gross Unrealized Gains, due within one year		17
Gross Unrealized Losses, due within one year		(2)
Estimated Fair Value, due within one year		$ 66,256