Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Amortized Cost	$ 461,085	$ 459,145
Gross Unrealized Gains	878	139
Gross Unrealized Losses	(115)	(28)
Estimated Fair Value	461,848	459,256
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	270,577	314,292
Gross Unrealized Gains	174	74
Gross Unrealized Losses	(115)	(23)
Estimated Fair Value	270,636	314,343
U.S. Government securities
Marketable Securities [Line Items]
Amortized Cost	134,685	78,612
Gross Unrealized Gains	608	48
Gross Unrealized Losses	0	(3)
Estimated Fair Value	135,293	78,657
Certificates of deposit
Marketable Securities [Line Items]
Amortized Cost	45,836	66,241
Gross Unrealized Gains	84	17
Gross Unrealized Losses	0	(2)
Estimated Fair Value	45,920	$ 66,256
Other debt securities
Marketable Securities [Line Items]
Amortized Cost	9,987
Gross Unrealized Gains	12
Gross Unrealized Losses	0
Estimated Fair Value	$ 9,999

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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