Cash, Cash Equivalents, Marketable Securities, and Restricted Cash - Components (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Cash, Money Market Funds, and Marketable Securities
Cash and cash equivalents	$ 210,631	$ 142,837	$ 166,319
Fair Value, Cash balances	210,631	142,837
Cost, available-for-sale securities	298,309	309,665
Gross unrealized gain, available-for-sale securities	159	255
Gross unrealized loss, available-for-sale securities	(17)	(17)
Fair value, Available-for-sale debt securities	298,451	309,903
Cost, Cash balances and available-for-sale securities	508,940	452,502
Gross unrealized gain, Cash balances and available-for-sale securities	159	255
Gross unrealized loss, Cash balances and available-for-sale securities	(17)	(17)
Fair value, Cash balances and available-for-sale securities	509,082	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	66,055	145,875
Corporate debt securities, Gross Unrealized Gain	60	121
Corporate debt securities, Gross Unrealized Loss	(5)	(5)
Corporate debt securities, Fair Value	66,110	145,991
Commercial paper
Cash, Money Market Funds, and Marketable Securities
Cost, available-for-sale securities	195,030	73,659
Gross unrealized gain, available-for-sale securities	36	53
Gross unrealized loss, available-for-sale securities	(12)	(2)
Fair value, Available-for-sale debt securities	195,054	73,710
Asset-backed securities
Cash, Money Market Funds, and Marketable Securities
Corporate debt securities, Cost	16,541	77,731
Corporate debt securities, Gross Unrealized Gain	61	79
Corporate debt securities, Gross Unrealized Loss	0	0
Corporate debt securities, Fair Value	16,602	77,810
U.S. government agency bonds
Cash, Money Market Funds, and Marketable Securities
Corporate debt securities, Cost	20,282	11,999
Corporate debt securities, Gross Unrealized Gain	2	2
Corporate debt securities, Gross Unrealized Loss	0	(10)
Corporate debt securities, Fair Value	20,284	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