Assets (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014
Total available-for-sale securities
Amortized Cost		$ 88,930,000	$ 120,537,000
Gross Unrealized Gains		70,000	47,000
Gross Unrealized Losses		(3,000)	(12,000)
Estimated Fair Value		88,997,000	120,572,000
Available-for-sale securities in a loss position
Estimated Fair value		28,083,000	48,070,000
Gross Unrealized Losses		(3,000)	(12,000)
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		49,028,000
Due after one year through two years		39,902,000
Gross realized gains on available-for-sale securities	0
Estimated fair value of contractual maturities of available-for-sale securities
Due in one year or less		49,055,000
Due in 1-2 years		39,942,000
Total available-for-sale securities		88,997,000	120,572,000
U.S. treasuries
Total available-for-sale securities
Amortized Cost		59,692,000	81,506,000
Gross Unrealized Gains		42,000	27,000
Gross Unrealized Losses		(2,000)	(6,000)
Estimated Fair Value		59,732,000	81,527,000
Available-for-sale securities in a loss position
Estimated Fair value		22,001,000	35,001,000
Gross Unrealized Losses		(2,000)	(6,000)
Government agency securities
Total available-for-sale securities
Amortized Cost		4,998,000
Gross Unrealized Gains		1,000
Estimated Fair Value		4,999,000
Corporate debt
Total available-for-sale securities
Amortized Cost		24,240,000	39,031,000
Gross Unrealized Gains		27,000	20,000
Gross Unrealized Losses		(1,000)	(6,000)
Estimated Fair Value		24,266,000	39,045,000
Available-for-sale securities in a loss position
Estimated Fair value		6,082,000	13,069,000
Gross Unrealized Losses		$ (1,000)	$ (6,000)