Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Investments	$ 29,303	$ 22,511
Measured at fair value | Level 1
Fair Value of Financial Instruments
Investments	33,137	53,511
Measured at fair value | Level 1 | Money market funds
Fair Value of Financial Instruments
Investments	33,137	53,511
Measured at fair value | Level 2
Fair Value of Financial Instruments
Investments	30,994	22,511
Measured at fair value | Level 2 | Federal agency securities
Fair Value of Financial Instruments
Investments		12,301
Measured at fair value | Level 2 | Commercial paper
Fair Value of Financial Instruments
Investments	10,247	8,249
Measured at fair value | Level 2 | Corporate debt securities
Fair Value of Financial Instruments
Investments	20,747	1,961
Measured at fair value | Total
Fair Value of Financial Instruments
Investments	64,131	76,022
Measured at fair value | Total | Money market funds
Fair Value of Financial Instruments
Investments	33,137	53,511
Measured at fair value | Total | Federal agency securities
Fair Value of Financial Instruments
Investments		12,301
Measured at fair value | Total | Commercial paper
Fair Value of Financial Instruments
Investments	10,247	8,249
Measured at fair value | Total | Corporate debt securities
Fair Value of Financial Instruments
Investments	$ 20,747	$ 1,961