Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Total available-for-sale securities
Amortized Cost	$ 120,537,000	$ 281,489,000
Gross Unrealized Gains	47,000	86,000
Gross Unrealized Losses	(12,000)	(37,000)
Estimated Fair Value	120,572,000	281,538,000
Available-for-sale securities in a loss position
Estimated Fair value	48,070,000	66,722,000
Gross Unrealized Losses	(12,000)	(37,000)
Investments that had been in a continuous loss position for more than 12 months	0	0
Amortized costs of contractual maturities of available-for-sale securities
Due in one year or less	74,710,000
Due after one year through two years	45,827,000
Gross realized gains on available-for-sale securities	100,000	100,000
Gross realized losses on available-for-sale securities	0	0	0
Estimated fair value of contractual maturities of available-for-sale securities
Due in one year or less	74,718,000
Due after one year through two years	45,854,000
Total available-for-sale securities	120,572,000	281,538,000
U.S. treasuries
Total available-for-sale securities
Amortized Cost	81,506,000	130,956,000
Gross Unrealized Gains	27,000	22,000
Gross Unrealized Losses	(6,000)	(1,000)
Estimated Fair Value	81,527,000	130,977,000
Available-for-sale securities in a loss position
Estimated Fair value	35,001,000	29,068,000
Gross Unrealized Losses	(6,000)	(1,000)
Government agency securities
Total available-for-sale securities
Amortized Cost		61,004,000
Gross Unrealized Gains		9,000
Estimated Fair Value		61,013,000
Corporate debt
Total available-for-sale securities
Amortized Cost	39,031,000	77,582,000
Gross Unrealized Gains	20,000	55,000
Gross Unrealized Losses	(6,000)	(36,000)
Estimated Fair Value	39,045,000	77,601,000
Available-for-sale securities in a loss position
Estimated Fair value	13,069,000	37,654,000
Gross Unrealized Losses	(6,000)	(36,000)
Commercial paper
Total available-for-sale securities
Amortized Cost		11,947,000
Estimated Fair Value		$ 11,947,000