Investments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Short-term investments:
Amortized Cost	$ 62,933,000		$ 62,933,000		$ 154,011,000
Gross Unrealized Gains	21,000		21,000		3,000
Gross Unrealized Losses	(12,000)		(12,000)		(69,000)
Estimated Fair Value	62,942,000		62,942,000		153,945,000
Long-term investments:
Amortized Cost	73,296,000		73,296,000		151,137,000
Gross Unrealized Gains	45,000		45,000		32,000
Gross Unrealized Losses	(97,000)		(97,000)		(333,000)
Estimated Fair Value	73,244,000		73,244,000		150,836,000
Amortized Cost	136,229,000		136,229,000		305,148,000
Gross Unrealized Gains	66,000		66,000		35,000
Gross Unrealized Losses	(109,000)		(109,000)		(402,000)
Estimated Fair Value	136,186,000		136,186,000		304,781,000
Other-than-temporary impairment losses	0	$ 0	0	$ 0
Corporate debt securities
Short-term investments:
Amortized Cost	47,494,000		47,494,000		106,430,000
Gross Unrealized Gains	21,000		21,000		3,000
Gross Unrealized Losses	(12,000)		(12,000)		(69,000)
Estimated Fair Value	47,503,000		47,503,000		106,364,000
Long-term investments:
Amortized Cost	63,918,000		63,918,000		139,742,000
Gross Unrealized Gains	45,000		45,000		32,000
Gross Unrealized Losses	(52,000)		(52,000)		(281,000)
Estimated Fair Value	63,911,000		63,911,000		139,493,000
U.S. treasury and government agency securities
Short-term investments:
Amortized Cost					1,021,000
Gross Unrealized Gains					0
Gross Unrealized Losses					0
Estimated Fair Value					1,021,000
Long-term investments:
Amortized Cost	9,378,000		9,378,000		11,395,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(45,000)		(45,000)		(52,000)
Estimated Fair Value	9,333,000		9,333,000		11,343,000
Commercial paper
Short-term investments:
Amortized Cost	3,990,000		3,990,000		40,560,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	3,990,000		3,990,000		40,560,000
Certificates of deposit
Short-term investments:
Amortized Cost	11,449,000		11,449,000		6,000,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Estimated Fair Value	$ 11,449,000		$ 11,449,000		$ 6,000,000