Marketable Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Short-term marketable securities:
Amortized Cost	$ 78,473,000		$ 78,473,000		$ 70,406,000
Gross Unrealized Gains	1,000		1,000		0
Gross Unrealized Losses	(203,000)		(203,000)		(81,000)
Estimated Fair Value	78,271,000		78,271,000		70,325,000
Long-term marketable securities:
Amortized Cost	62,753,000		62,753,000		66,663,000
Gross Unrealized Gains	2,000		2,000		1,000
Gross Unrealized Losses	(658,000)		(658,000)		(396,000)
Estimated Fair Value	62,097,000		62,097,000		66,268,000
Amortized Cost	141,226,000		141,226,000		137,069,000
Gross Unrealized Gains	3,000		3,000		1,000
Gross Unrealized Losses	(861,000)		(861,000)		(477,000)
Estimated Fair Value	140,368,000		140,368,000		136,593,000
Other-than-temporary impairment losses	0	$ 0	0	$ 0
Corporate debt securities
Short-term marketable securities:
Amortized Cost	53,862,000		53,862,000		57,257,000
Gross Unrealized Gains	1,000		1,000		0
Gross Unrealized Losses	(174,000)		(174,000)		(68,000)
Estimated Fair Value	53,689,000		53,689,000		57,189,000
Long-term marketable securities:
Amortized Cost	55,017,000		55,017,000		59,282,000
Gross Unrealized Gains	2,000		2,000		1,000
Gross Unrealized Losses	(580,000)		(580,000)		(320,000)
Estimated Fair Value	54,439,000		54,439,000		58,963,000
Certificates of deposit
Short-term marketable securities:
Amortized Cost	13,000,000		13,000,000		9,151,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	13,000,000		13,000,000		9,151,000
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	6,149,000		6,149,000		1,999,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(29,000)		(29,000)		(13,000)
Estimated Fair Value	6,120,000		6,120,000		1,986,000
Long-term marketable securities:
Amortized Cost	6,236,000		6,236,000		7,381,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(78,000)		(78,000)		(76,000)
Estimated Fair Value	6,158,000		6,158,000		7,305,000
Commercial paper
Short-term marketable securities:
Amortized Cost	5,462,000		5,462,000		1,999,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	$ 5,462,000		$ 5,462,000		$ 1,999,000