Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities, Amortized Cost	$ 203,432	$ 131,699
Total cash equivalents and marketable securities, Gross Unrealized Gains	19	27
Total cash equivalents and marketable securities, Gross Unrealized Losses	(1)	(8)
Total cash equivalents and marketable securities, Fair Value	203,450	131,718
Corporate debt securities
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities, Amortized Cost	4,893	33,653
Total cash equivalents and marketable securities, Gross Unrealized Gains	5	11
Total cash equivalents and marketable securities, Gross Unrealized Losses		(2)
Total cash equivalents and marketable securities, Fair Value	4,898	33,662
U.S. Treasury and agency securities
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities, Amortized Cost	44,980	40,798
Total cash equivalents and marketable securities, Gross Unrealized Gains	8	14
Total cash equivalents and marketable securities, Gross Unrealized Losses	(1)	(2)
Total cash equivalents and marketable securities, Fair Value	44,987	40,810
Money Market Funds
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities, Amortized Cost	139,027	12,964
Total cash equivalents and marketable securities, Fair Value	139,027	12,964
Commercial Paper
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities, Amortized Cost	14,532	44,284
Total cash equivalents and marketable securities, Gross Unrealized Gains	6	2
Total cash equivalents and marketable securities, Gross Unrealized Losses		(4)
Total cash equivalents and marketable securities, Fair Value	$ 14,538	$ 44,282