Marketable Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Short-term marketable securities:
Amortized Cost	$ 51,343,000		$ 51,343,000		$ 74,826,000
Gross Unrealized Gains	130,000		130,000		0
Gross Unrealized Losses	(12,000)		(12,000)		(270,000)
Estimated Fair Value	51,461,000		51,461,000		74,556,000
Long-term marketable securities:
Amortized Cost	20,076,000		20,076,000		66,772,000
Gross Unrealized Gains	207,000		207,000		52,000
Gross Unrealized Losses	0		0		(465,000)
Estimated Fair Value	20,283,000		20,283,000		66,359,000
Amortized Cost	71,419,000		71,419,000		141,598,000
Gross Unrealized Gains	337,000		337,000		52,000
Gross Unrealized Losses	(12,000)		(12,000)		(735,000)
Estimated Fair Value	71,744,000		71,744,000		140,915,000
Other-than-temporary impairment losses	0	$ 0	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	41,596,000		41,596,000		51,184,000
Gross Unrealized Gains	130,000		130,000		0
Gross Unrealized Losses	(9,000)		(9,000)		(236,000)
Estimated Fair Value	41,717,000		41,717,000		50,948,000
Long-term marketable securities:
Amortized Cost	20,076,000		20,076,000		62,530,000
Gross Unrealized Gains	207,000		207,000		52,000
Gross Unrealized Losses	0		0		(433,000)
Estimated Fair Value	20,283,000		20,283,000		62,149,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	3,500,000		3,500,000		12,000,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	3,500,000		3,500,000		12,000,000
Long-term marketable securities:
Amortized Cost					1,500,000
Gross Unrealized Gains					0
Gross Unrealized Losses					0
Estimated Fair Value					1,500,000
U.S. treasury and government agency securities
Short-term marketable securities:
Amortized Cost	6,247,000		6,247,000		7,647,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(3,000)		(3,000)		(34,000)
Estimated Fair Value	6,244,000		6,244,000		7,613,000
Long-term marketable securities:
Amortized Cost					2,742,000
Gross Unrealized Gains					0
Gross Unrealized Losses					(32,000)
Estimated Fair Value					2,710,000
Commercial paper
Short-term marketable securities:
Amortized Cost	0		0		3,995,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	$ 0		$ 0		$ 3,995,000