Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Short-term marketable securities:
Amortized Cost	$ 57,259,000		$ 74,826,000
Gross Unrealized Gains	17,000		0
Gross Unrealized Losses	(124,000)		(270,000)
Estimated Fair Value	57,152,000		74,556,000
Long-term marketable securities:
Amortized Cost	71,234,000		66,772,000
Gross Unrealized Gains	362,000		52,000
Gross Unrealized Losses	(155,000)		(465,000)
Estimated Fair Value	71,441,000		66,359,000
Amortized Cost	128,493,000		141,598,000
Gross Unrealized Gains	379,000		52,000
Gross Unrealized Losses	(279,000)		(735,000)
Estimated Fair Value	128,593,000		140,915,000
Other-than-temporary impairment losses	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	39,368,000		51,184,000
Gross Unrealized Gains	17,000		0
Gross Unrealized Losses	(90,000)		(236,000)
Estimated Fair Value	39,295,000		50,948,000
Long-term marketable securities:
Amortized Cost	69,734,000		62,530,000
Gross Unrealized Gains	362,000		52,000
Gross Unrealized Losses	(155,000)		(433,000)
Estimated Fair Value	69,941,000		62,149,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	10,000,000		7,647,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		(34,000)
Estimated Fair Value	10,000,000		7,613,000
Long-term marketable securities:
Amortized Cost	1,500,000		1,500,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	1,500,000		1,500,000
U.S. treasury and government agency securities
Short-term marketable securities:
Amortized Cost	6,391,000		3,995,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(34,000)		0
Estimated Fair Value	6,357,000		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		12,000,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	$ 1,500,000		$ 12,000,000