Marketable Securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Short-term marketable securities:
Amortized Cost	$ 63,304,000		$ 63,304,000		$ 74,826,000
Gross Unrealized Gains	122,000		122,000		0
Gross Unrealized Losses	(63,000)		(63,000)		(270,000)
Estimated Fair Value	63,363,000		63,363,000		74,556,000
Long-term marketable securities:
Amortized Cost	46,739,000		46,739,000		66,772,000
Gross Unrealized Gains	500,000		500,000		52,000
Gross Unrealized Losses	(19,000)		(19,000)		(465,000)
Estimated Fair Value	47,220,000		47,220,000		66,359,000
Amortized Cost	110,043,000		110,043,000		141,598,000
Gross Unrealized Gains	622,000		622,000		52,000
Gross Unrealized Losses	(82,000)		(82,000)		(735,000)
Estimated Fair Value	110,583,000		110,583,000		140,915,000
Other-than-temporary impairment losses	0	$ 0	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	47,410,000		47,410,000		51,184,000
Gross Unrealized Gains	122,000		122,000		0
Gross Unrealized Losses	(52,000)		(52,000)		(236,000)
Estimated Fair Value	47,480,000		47,480,000		50,948,000
Long-term marketable securities:
Amortized Cost	46,739,000		46,739,000		62,530,000
Gross Unrealized Gains	500,000		500,000		52,000
Gross Unrealized Losses	(19,000)		(19,000)		(433,000)
Estimated Fair Value	47,220,000		47,220,000		62,149,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	8,000,000		8,000,000		7,647,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		(34,000)
Estimated Fair Value	8,000,000		8,000,000		7,613,000
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	6,394,000		6,394,000		3,995,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(11,000)		(11,000)		0
Estimated Fair Value	6,383,000		6,383,000		3,995,000
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	1,500,000		1,500,000		12,000,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	$ 1,500,000		$ 1,500,000		$ 12,000,000