Fair Value Measurements (Details) (Fair Value Measurements Recurring [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair value of assets and liabilities measured on a recurring basis
Transfers or reclassifications of securities from Level 1 to Level 2	$ 0
Transfers or reclassifications of securities from Level 2 to Level 1	0
Estimate Of Fair Value Fair Value Disclosure [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	381,242	205,570
Total Liabilities	15,741	14,550
Estimate Of Fair Value Fair Value Disclosure [Member] | Acquisition Related Contingent Consideration [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	15,741	14,550
Estimate Of Fair Value Fair Value Disclosure [Member] | Money Market Funds [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,826	18,767
Estimate Of Fair Value Fair Value Disclosure [Member] | Corporate Debt Securities [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	131,274	134,123
Estimate Of Fair Value Fair Value Disclosure [Member] | U S Treasury And Government [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	51,642	52,680
Estimate Of Fair Value Fair Value Disclosure [Member] | Commercial Paper Not Included With Cash And Cash Equivalents [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	1,500
Fair Value Inputs Level1 [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,826	18,767
Fair Value Inputs Level1 [Member] | Money Market Funds [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	196,826	18,767
Fair Value Inputs Level2 [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	184,416	186,803
Fair Value Inputs Level2 [Member] | Corporate Debt Securities [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	131,274	134,123
Fair Value Inputs Level2 [Member] | U S Treasury And Government [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	51,642	52,680
Fair Value Inputs Level2 [Member] | Commercial Paper Not Included With Cash And Cash Equivalents [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Assets	1,500
Fair Value Inputs Level3 [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	15,741	14,550
Fair Value Inputs Level3 [Member] | Acquisition Related Contingent Consideration [Member]
Fair value of assets and liabilities measured on a recurring basis
Total Liabilities	$ 15,741	$ 14,550