Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Investments	$ 48,981	$ 22,511
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level1 [Member]
Fair Value of Financial Instruments
Investments	8,840	53,511
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level1 [Member] | Money Market Funds [Member]
Fair Value of Financial Instruments
Investments	8,840	53,511
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level2 [Member]
Fair Value of Financial Instruments
Investments	49,734	22,511
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level2 [Member] | U S Government Corporations And Agencies Securities [Member]
Fair Value of Financial Instruments
Investments	15,094	12,301
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level2 [Member] | Commercial Paper [Member]
Fair Value of Financial Instruments
Investments	10,249	8,249
Fair Value Measurements Recurring [Member] | Fair Value Inputs Level2 [Member] | Corporate Debt Securities [Member]
Fair Value of Financial Instruments
Investments	24,391	1,961
Fair Value Measurements Recurring [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value of Financial Instruments
Investments	58,574	76,022
Fair Value Measurements Recurring [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Money Market Funds [Member]
Fair Value of Financial Instruments
Investments	8,840	53,511
Fair Value Measurements Recurring [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | U S Government Corporations And Agencies Securities [Member]
Fair Value of Financial Instruments
Investments	15,094	12,301
Fair Value Measurements Recurring [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Commercial Paper [Member]
Fair Value of Financial Instruments
Investments	10,249	8,249
Fair Value Measurements Recurring [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Corporate Debt Securities [Member]
Fair Value of Financial Instruments
Investments	$ 24,391	$ 1,961