Assets (Details 2) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Total available-for-sale securities
Amortized Cost	$ 68,834	$ 120,537
Gross Unrealized Gains	44	47
Gross Unrealized Losses	(1)	(12)
Estimated Fair Value	68,877	120,572
Available-for-sale securities in a loss position
Estimated Fair value	3,139	48,070
Gross Unrealized Losses	(1)	(12)
Investments that had been in a continuous loss position for more than 12 months	0	0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	54,693	81,506
Gross Unrealized Gains	38	27
Gross Unrealized Losses	0	(6)
Estimated Fair Value	54,731	81,527
Available-for-sale securities in a loss position
Estimated Fair value	2,000	35,001
Gross Unrealized Losses	0	(6)
Government agency securities
Total available-for-sale securities
Amortized Cost	2,999
Estimated Fair Value	2,999
Corporate debt
Total available-for-sale securities
Amortized Cost	11,142	39,031
Gross Unrealized Gains	6	20
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	11,147	39,045
Available-for-sale securities in a loss position
Estimated Fair value	1,139	13,069
Gross Unrealized Losses	$ (1)	$ (6)