Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash	$ 23,688	$ 43,058
Total cash and cash equivalents	136,871	62,070
Marketable securities, amortized cost	252,725	188,706
Marketable securities, gross unrealized gains	653	381
Marketable securities, gross unrealized losses	0	(2)
Marketable securities, estimated fair value	253,378	189,085
Total cash, cash equivalents and marketable securities, amortized cost	389,596	250,776
Total cash, cash equivalents and marketable securities	390,249	251,155
Amortized Cost
Due within one year, amortized cost	245,025
Due between one and two years, amortized cost	7,700
Marketable securities, amortized cost	252,725
Fair Value
Due within one year, estimated fair value	245,635
Due between one and two years, estimated fair value	7,743
Marketable securities, estimated fair value	253,378
Prepaid expenses and other current assets
Fair Value
Accrued interest receivable	400	600
U.S. Treasuries
Cash and Cash Equivalents [Line Items]
Cash equivalents:	11,300
Marketable securities, amortized cost	55,677
Marketable securities, gross unrealized gains	3
Marketable securities, gross unrealized losses	0
Marketable securities, estimated fair value	55,680
Money market funds
Cash and Cash Equivalents [Line Items]
Cash equivalents:	61,840	11,527
Certificate of deposits
Cash and Cash Equivalents [Line Items]
Cash equivalents:	10,900
Marketable securities, amortized cost	42,744	41,302
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	42,744	41,302
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash equivalents:	29,143	7,485
Marketable securities, amortized cost	94,567	73,366
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	94,567	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	13
Marketable securities, gross unrealized losses	0
Marketable securities, estimated fair value	6,213
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	53,537	74,038
Marketable securities, gross unrealized gains	637	381
Marketable securities, gross unrealized losses	0	(2)
Marketable securities, estimated fair value	$ 54,174	$ 74,417