Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash and cash equivalents:
AMORTIZED COST	$ 134,179	$ 143,940	$ 148,868	$ 202,818
Investments:
AMORTIZED COST	130,683	165,342
GROSS UNREALIZED GAINS	85	0
GROSS UNREALIZED LOSSES	(216)	(92)
FAIR VALUE	130,552	165,250
Total cash, cash equivalents and investments, Amortized Cost	264,851	309,282
Total cash, cash equivalents and investments, Gross Unrealized Gains	96	0
Total cash, cash equivalents and investments, Gross Unrealized Losses	(216)	(92)
Total cash, cash equivalents and investments, Fair Value	264,731	309,190
Cash and cash equivalents
Cash and cash equivalents:
AMORTIZED COST	134,168	143,940
GROSS UNREALIZED GAINS	11	0
GROSS UNREALIZED LOSSES	0	0
FAIR VALUE	134,179	143,940
Commercial paper (due within 1 year)
Investments:
AMORTIZED COST	44,751	64,629
GROSS UNREALIZED GAINS	16	0
GROSS UNREALIZED LOSSES	(28)	(7)
FAIR VALUE	44,739	64,622
Corporate bonds (due within 1 year)
Investments:
AMORTIZED COST	55,904	60,640
GROSS UNREALIZED GAINS	13	0
GROSS UNREALIZED LOSSES	(188)	(76)
FAIR VALUE	55,729	60,564
U.S. Government and government agencies (due within 1 year)
Investments:
AMORTIZED COST	30,028	40,073
GROSS UNREALIZED GAINS	56	0
GROSS UNREALIZED LOSSES	0	(9)
FAIR VALUE	$ 30,084	$ 40,064