Cash, Cash Equivalents, Restricted Cash and Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	$ 178,729	$ 473,284
Gross Unrealized Gains	475	5
Gross Unrealized Losses	0	(2,565)
Fair Value	179,204	470,724
Commercial paper
Restricted Cash and Cash Equivalents Items [Line Items]
Cost		14,548
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		14,548
U.S. government securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	105,453	291,157
Gross Unrealized Gains	249	0
Gross Unrealized Losses	0	(1,581)
Fair Value	105,702	289,576
Corporate bonds, asset backed and other securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cost	73,276	167,579
Gross Unrealized Gains	226	5
Gross Unrealized Losses	0	(984)
Fair Value	73,502	166,600
Cash and Cash Equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Fair Value	0	0
Short-term Investments
Restricted Cash and Cash Equivalents Items [Line Items]
Fair Value	$ 179,204	$ 470,724