Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term investments:
Amortized Cost, short-term investments	$ 74,826,000	$ 70,406,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	(270,000)	(81,000)
Estimated Fair Value, short-term investments	74,556,000	70,325,000
Long-term investments:
Amortized Cost, long-term investments	66,772,000	66,663,000
Gross Unrealized Gains, long-term investments	52,000	1,000
Gross Unrealized Losses, long-term investments	(465,000)	(396,000)
Estimated Fair Value, long-term investments	66,359,000	66,268,000
Total investments
Amortized Cost, total	141,598,000	137,069,000
Gross Unrealized Gains, total	52,000	1,000
Gross Unrealized Losses, total	(735,000)	(477,000)
Estimated Fair Value, total	140,915,000	136,593,000
Other-than-temporary impairment losses	0	0	$ 0
Corporate debt securities
Short-term investments:
Amortized Cost, short-term investments	51,184,000	57,257,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	(236,000)	(68,000)
Estimated Fair Value, short-term investments	50,948,000	57,189,000
Long-term investments:
Amortized Cost, long-term investments	62,530,000	59,282,000
Gross Unrealized Gains, long-term investments	52,000	1,000
Gross Unrealized Losses, long-term investments	(433,000)	(320,000)
Estimated Fair Value, long-term investments	62,149,000	58,963,000
U.S. treasury and government agency securities
Short-term investments:
Amortized Cost, short-term investments	7,647,000	1,999,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	(34,000)	(13,000)
Estimated Fair Value, short-term investments	7,613,000	1,986,000
Long-term investments:
Amortized Cost, long-term investments	2,742,000	7,381,000
Gross Unrealized Gains, long-term investments	0	0
Gross Unrealized Losses, long-term investments	(32,000)	(76,000)
Estimated Fair Value, long-term investments	2,710,000	7,305,000
Commercial paper
Short-term investments:
Amortized Cost, short-term investments	3,995,000	1,999,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	0
Estimated Fair Value, short-term investments	3,995,000	1,999,000
Certificates of deposit
Short-term investments:
Amortized Cost, short-term investments	12,000,000	9,151,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	0
Estimated Fair Value, short-term investments	12,000,000	$ 9,151,000
Long-term investments:
Amortized Cost, long-term investments	1,500,000
Gross Unrealized Gains, long-term investments	0
Gross Unrealized Losses, long-term investments	0
Estimated Fair Value, long-term investments	$ 1,500,000