Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash	$ 219,469	$ 43,058
Total cash and cash equivalents	297,460	62,070
Marketable securities, amortized cost	227,309	188,706
Marketable securities, gross unrealized gains	398	381
Marketable securities, gross unrealized losses	0	(2)
Marketable securities, estimated fair value	227,707	189,085
Total cash, cash equivalents and marketable securities, amortized cost	524,769	250,776
Total cash, cash equivalents and marketable securities	525,167	251,155
Amortized Cost
Due within one year, amortized cost	216,609
Due between one and two years, amortized cost	10,700
Marketable securities, amortized cost	227,309
Fair Value
Due within one year, estimated fair value	216,962
Due between one and two years, estimated fair value	10,745
Marketable securities, estimated fair value	227,707
Prepaid expenses and other current assets
Fair Value
Accrued interest receivable	600	600
U.S. Treasuries
Cash and Cash Equivalents [Line Items]
Cash equivalents:	13,799
Marketable securities, amortized cost	27,894
Marketable securities, gross unrealized gains	2
Marketable securities, gross unrealized losses	0
Marketable securities, estimated fair value	27,896
Money market funds
Cash and Cash Equivalents [Line Items]
Cash equivalents:	39,536	11,527
Certificate of deposits
Cash and Cash Equivalents [Line Items]
Cash equivalents:	3,008
Marketable securities, amortized cost	41,284	41,302
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	41,284	41,302
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash equivalents:	21,648	7,485
Marketable securities, amortized cost	101,951	73,366
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	101,951	73,366
U.S. Government-sponsored enterprises debt securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	6,200
Marketable securities, gross unrealized gains	19
Marketable securities, gross unrealized losses	0
Marketable securities, estimated fair value	6,219
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	49,980	74,038
Marketable securities, gross unrealized gains	377	381
Marketable securities, gross unrealized losses	0	(2)
Marketable securities, estimated fair value	$ 50,357	$ 74,417