Marketable Securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Short-term marketable securities:
Amortized Cost	$ 78,203,000		$ 78,203,000		$ 70,406,000
Gross Unrealized Gains	1,000		1,000		0
Gross Unrealized Losses	(240,000)		(240,000)		(81,000)
Estimated Fair Value	77,964,000		77,964,000		70,325,000
Long-term marketable securities:
Amortized Cost	61,462,000		61,462,000		66,663,000
Gross Unrealized Gains	5,000		5,000		1,000
Gross Unrealized Losses	(759,000)		(759,000)		(396,000)
Estimated Fair Value	60,708,000		60,708,000		66,268,000
Amortized Cost	139,665,000		139,665,000		137,069,000
Gross Unrealized Gains	6,000		6,000		1,000
Gross Unrealized Losses	(999,000)		(999,000)		(477,000)
Estimated Fair Value	138,672,000		138,672,000		136,593,000
Other-than-temporary impairment losses	0	$ 0	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	55,303,000		55,303,000		57,257,000
Gross Unrealized Gains	1,000		1,000		0
Gross Unrealized Losses	(197,000)		(197,000)		(68,000)
Estimated Fair Value	55,107,000		55,107,000		57,189,000
Long-term marketable securities:
Amortized Cost	55,079,000		55,079,000		59,282,000
Gross Unrealized Gains	5,000		5,000		1,000
Gross Unrealized Losses	(685,000)		(685,000)		(320,000)
Estimated Fair Value	54,399,000		54,399,000		58,963,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	9,450,000		9,450,000		9,151,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	9,450,000		9,450,000		9,151,000
U.S. treasury and government agency securities
Short-term marketable securities:
Amortized Cost	5,998,000		5,998,000		1,999,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(43,000)		(43,000)		(13,000)
Estimated Fair Value	5,955,000		5,955,000		1,986,000
Long-term marketable securities:
Amortized Cost	6,383,000		6,383,000		7,381,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(74,000)		(74,000)		(76,000)
Estimated Fair Value	6,309,000		6,309,000		7,305,000
Commercial paper
Short-term marketable securities:
Amortized Cost	7,452,000		7,452,000		1,999,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	$ 7,452,000		$ 7,452,000		$ 1,999,000