Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term investments:
Amortized Cost, short-term investments	$ 70,406,000	$ 154,011,000
Gross Unrealized Gains, short-term investments	0	3,000
Gross Unrealized Losses, short-term investments	(81,000)	(69,000)
Estimated Fair Value, short-term investments	70,325,000	153,945,000
Long-term investments:
Amortized Cost, long-term investments	66,663,000	151,137,000
Gross Unrealized Gains, long-term investments	1,000	32,000
Gross Unrealized Losses, long-term investments	(396,000)	(333,000)
Estimated Fair Value, long-term investments	66,268,000	150,836,000
Total investments
Amortized Cost, total	137,069,000	305,148,000
Gross Unrealized Gains, total	1,000	35,000
Gross Unrealized Losses, total	(477,000)	(402,000)
Estimated Fair Value, total	136,593,000	304,781,000
Other-than-temporary impairment losses	0	0	$ 0
Corporate debt securities
Short-term investments:
Amortized Cost, short-term investments	57,257,000	106,430,000
Gross Unrealized Gains, short-term investments	0	3,000
Gross Unrealized Losses, short-term investments	(68,000)	(69,000)
Estimated Fair Value, short-term investments	57,189,000	106,364,000
Long-term investments:
Amortized Cost, long-term investments	59,282,000	139,742,000
Gross Unrealized Gains, long-term investments	1,000	32,000
Gross Unrealized Losses, long-term investments	(320,000)	(281,000)
Estimated Fair Value, long-term investments	58,963,000	139,493,000
U.S. treasury and government agency securities
Short-term investments:
Amortized Cost, short-term investments	1,999,000	1,021,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	(13,000)	0
Estimated Fair Value, short-term investments	1,986,000	1,021,000
Long-term investments:
Amortized Cost, long-term investments	7,381,000	11,395,000
Gross Unrealized Gains, long-term investments	0	0
Gross Unrealized Losses, long-term investments	(76,000)	(52,000)
Estimated Fair Value, long-term investments	7,305,000	11,343,000
Commercial paper
Short-term investments:
Amortized Cost, short-term investments	1,999,000	40,560,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	0
Estimated Fair Value, short-term investments	1,999,000	40,560,000
Certificates of deposit
Short-term investments:
Amortized Cost, short-term investments	9,151,000	6,000,000
Gross Unrealized Gains, short-term investments	0	0
Gross Unrealized Losses, short-term investments	0	0
Estimated Fair Value, short-term investments	$ 9,151,000	$ 6,000,000