Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Fair Value of Financial Instruments
Cash and cash equivalents, cost	$ 3,040	$ 4,364	$ 8,172	$ 2,529
Cash and cash equivalents, fair value	3,040	4,364
Estimated fair values of financial instruments
Unrealized Gains	21	23
Unrealized Losses	(11)	(10)
Short-term investments
Estimated fair values of financial instruments
Cost	27,975	32,183
Unrealized Gains	20	14
Unrealized Losses	(1)	(5)
Fair Value	27,994	32,192
Short-term investments | U.S. government debt securities
Estimated fair values of financial instruments
Cost	11,667	19,944
Unrealized Gains	5	11
Unrealized Losses		(1)
Fair Value	11,672	19,954
Short-term investments | Corporate notes
Estimated fair values of financial instruments
Cost	12,524	7,245
Unrealized Gains	14	2
Unrealized Losses	(1)	(2)
Fair Value	12,537	7,245
Short-term investments | Certificates of deposit
Estimated fair values of financial instruments
Cost	3,784	4,994
Unrealized Gains	1	1
Unrealized Losses		(2)
Fair Value	3,785	4,993
Long-term investments
Estimated fair values of financial instruments
Cost	9,115	13,922
Unrealized Gains	1	9
Unrealized Losses	(10)	(5)
Fair Value	9,106	13,926
Long-term investments | U.S. government debt securities
Estimated fair values of financial instruments
Cost	2,800	3,016
Unrealized Losses	(2)
Fair Value	2,798	3,016
Long-term investments | Corporate notes
Estimated fair values of financial instruments
Cost	6,075	9,466
Unrealized Gains	1	9
Unrealized Losses	(8)	(1)
Fair Value	6,068	9,474
Long-term investments | Certificates of deposit
Estimated fair values of financial instruments
Cost	240	1,440
Unrealized Losses		(4)
Fair Value	$ 240	$ 1,436