Assets (Details 2) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Total available-for-sale securities
Amortized Cost	$ 82,337	$ 120,537
Gross Unrealized Gains	61	47
Gross Unrealized Losses	(1)	(12)
Estimated Fair Value	82,397	120,572
Available-for-sale securities in a loss position
Estimated Fair value	2,139	48,070
Gross Unrealized Losses	(1)	(12)
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	44,647
Due after one year through two years	37,690
Estimated fair value of contractual maturities of available-for-sale securities
Due in one year or less	44,668
Due in 1-2 years	37,729
U.S. treasuries
Total available-for-sale securities
Amortized Cost	59,693	81,506
Gross Unrealized Gains	47	27
Gross Unrealized Losses		(6)
Estimated Fair Value	59,740	81,527
Available-for-sale securities in a loss position
Estimated Fair value		35,001
Gross Unrealized Losses		(6)
Government agency securities
Total available-for-sale securities
Amortized Cost	5,000
Estimated Fair Value	5,000
Corporate debt
Total available-for-sale securities
Amortized Cost	17,644	39,031
Gross Unrealized Gains	14	20
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	17,657	39,045
Available-for-sale securities in a loss position
Estimated Fair value	2,139	13,069
Gross Unrealized Losses	$ (1)	$ (6)