Fair Value (Details) (Recurring Basis [Member], USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Financial assets measured at fair value on a recurring basis
Total	$ 110,904	$ 92,233
Level 1 [Member]
Financial assets measured at fair value on a recurring basis
Total	45,256	68,395
Level 1 [Member] | Money market funds [Member] | Cash and Cash Equivalents [Member]
Financial assets measured at fair value on a recurring basis
Total	45,256	68,395
Level 1 [Member] | Commercial Paper [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0
Level 1 [Member] | Corporate bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 1 [Member] | State, municipal and local government agencies bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 1 [Member] | U.S. federal government bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0
Level 2 [Member]
Financial assets measured at fair value on a recurring basis
Total	65,648	23,838
Level 2 [Member] | Money market funds [Member] | Cash and Cash Equivalents [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 2 [Member] | Commercial Paper [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	4,598
Level 2 [Member] | Corporate bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	18,297	11,907
Level 2 [Member] | State, municipal and local government agencies bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	34,167	11,931
Level 2 [Member] | U.S. federal government bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	8,586
Level 3 [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 3 [Member] | Money market funds [Member] | Cash and Cash Equivalents [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 3 [Member] | Commercial Paper [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0
Level 3 [Member] | Corporate bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 3 [Member] | State, municipal and local government agencies bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0	0
Level 3 [Member] | U.S. federal government bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	0
Money market funds [Member] | Cash and Cash Equivalents [Member]
Financial assets measured at fair value on a recurring basis
Total	45,256	68,395
Commercial Paper [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	4,598
Corporate bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	18,297	11,907
State, municipal and local government agencies bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	34,167	11,931
U.S. federal government bonds [Member] | Short-term Investments [Member]
Financial assets measured at fair value on a recurring basis
Total	$ 8,586