Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Components (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Cash, Money Market Funds, and Marketable Securities
Cash and cash equivalents	$ 164,573	$ 142,837	$ 220,578
Fair Value, Cash balances	164,573	142,837
Cost, available-for-sale securities	144,531	309,665
Gross unrealized gain, available-for-sale securities	490	255
Gross unrealized loss, available-for-sale securities	(4)	(17)
Fair value, Available-for-sale debt securities	145,017	309,903
Cost, Cash balances and available-for-sale securities	309,104	452,502
Gross unrealized gain, Cash balances and available-for-sale securities	490	255
Gross unrealized loss, Cash balances and available-for-sale securities	(4)	(17)
Fair value, Cash balances and available-for-sale securities	309,590	452,740
Marketable securities with maturity date greater than 12 months		9,500
Corporate debt securities
Cash, Money Market Funds, and Marketable Securities
Corporate debt securities, Cost	51,307	145,875
Corporate debt securities, Gross Unrealized Gain	225	121
Corporate debt securities, Gross Unrealized Loss	0	(5)
Corporate debt securities, Fair Value	51,532	145,991
Commercial paper
Cash, Money Market Funds, and Marketable Securities
Cost, available-for-sale securities	59,803	73,659
Gross unrealized gain, available-for-sale securities	103	53
Gross unrealized loss, available-for-sale securities	(4)	(2)
Fair value, Available-for-sale debt securities	59,902	73,710
Asset-backed securities
Cash, Money Market Funds, and Marketable Securities
Corporate debt securities, Cost	33,020	77,731
Corporate debt securities, Gross Unrealized Gain	162	79
Corporate debt securities, Gross Unrealized Loss	0	0
Corporate debt securities, Fair Value	33,182	77,810
U.S. government agency bonds
Cash, Money Market Funds, and Marketable Securities
Corporate debt securities, Cost		11,999
Corporate debt securities, Gross Unrealized Gain		2
Corporate debt securities, Gross Unrealized Loss		(10)
Corporate debt securities, Fair Value		11,991
Money market
Cash, Money Market Funds, and Marketable Securities
Cost, available-for-sale securities	350	350
Gross unrealized gain, available-for-sale securities	0	0
Gross unrealized loss, available-for-sale securities	0	0
Fair value, Available-for-sale debt securities	350	350
Certificates of deposit
Cash, Money Market Funds, and Marketable Securities
Cost, available-for-sale securities	51	51
Gross unrealized gain, available-for-sale securities	0	0
Gross unrealized loss, available-for-sale securities	0	0
Fair value, Available-for-sale debt securities	$ 51	$ 51