Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Total assets measured at fair value	$ 29,790	$ 22,511
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	9,663	53,511
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	9,663	53,511
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	29,790	22,511
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Corporations and Agencies Securities [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	13,770	12,301
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	1,500	8,249
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	14,520	1,961
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	39,453	76,022
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Money Market Funds [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	9,663	53,511
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | US Government Corporations and Agencies Securities [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	13,770	12,301
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Commercial Paper [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	1,500	8,249
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Corporate Debt Securities [Member]
Fair Value of Financial Instruments
Total assets measured at fair value	$ 14,520	$ 1,961