Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Short-term marketable securities:
Amortized Cost	$ 81,089,000		$ 70,406,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(272,000)		(81,000)
Estimated Fair Value	80,817,000		70,325,000
Long-term marketable securities:
Amortized Cost	58,789,000		66,663,000
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(787,000)		(396,000)
Estimated Fair Value	58,003,000		66,268,000
Amortized Cost	139,878,000		137,069,000
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(1,059,000)		(477,000)
Estimated Fair Value	138,820,000		136,593,000
Other-than-temporary impairment losses	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	58,981,000		57,257,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(215,000)		(68,000)
Estimated Fair Value	58,766,000		57,189,000
Long-term marketable securities:
Amortized Cost	52,408,000		59,282,000
Gross Unrealized Gains	1,000		1,000
Gross Unrealized Losses	(725,000)		(320,000)
Estimated Fair Value	51,684,000		58,963,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	10,150,000		9,151,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	10,150,000		9,151,000
U.S. treasury and government agency securities
Short-term marketable securities:
Amortized Cost	5,996,000		1,999,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(57,000)		(13,000)
Estimated Fair Value	5,939,000		1,986,000
Long-term marketable securities:
Amortized Cost	6,381,000		7,381,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(62,000)		(76,000)
Estimated Fair Value	6,319,000		7,305,000
Commercial paper
Short-term marketable securities:
Amortized Cost	5,962,000		1,999,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Estimated Fair Value	$ 5,962,000		$ 1,999,000