Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Cash and cash equivalents:
AMORTIZED COST	$ 151,299	$ 143,940	$ 109,363	$ 202,818
FAIR VALUE	151,299	143,940
Investments:
AMORTIZED COST	90,504	165,342
GROSS UNREALIZED GAINS	147	0
GROSS UNREALIZED LOSSES	(32)	(92)
FAIR VALUE	90,619	165,250
Total cash, cash equivalents and investments, Amortized Cost	241,801	309,282
Total cash, cash equivalents and investments, Gross Unrealized Gains	149	0
Total cash, cash equivalents and investments, Gross Unrealized Losses	(32)	(92)
Total cash, cash equivalents and investments, Fair Value	241,918	309,190
Cash and cash equivalents
Cash and cash equivalents:
AMORTIZED COST	151,297	143,940
GROSS UNREALIZED GAINS	2	0
GROSS UNREALIZED LOSSES	0	0
FAIR VALUE	151,299	143,940
Commercial paper (due within 1 year)
Investments:
AMORTIZED COST	39,958	64,629
GROSS UNREALIZED GAINS	77	0
GROSS UNREALIZED LOSSES	(7)	(7)
FAIR VALUE	40,028	64,622
Corporate bonds (due within 1 year)
Investments:
AMORTIZED COST	50,546	60,640
GROSS UNREALIZED GAINS	70	0
GROSS UNREALIZED LOSSES	(25)	(76)
FAIR VALUE	$ 50,591	60,564
U.S. Government and government agencies (due within 1 year)
Investments:
AMORTIZED COST		40,073
GROSS UNREALIZED GAINS		0
GROSS UNREALIZED LOSSES		(9)
FAIR VALUE		$ 40,064