Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Actively Quoted Markets for Identical Assets Level 1
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 11,451	$ 7,377
Actively Quoted Markets for Identical Assets Level 1 | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	10,160	7,377
Actively Quoted Markets for Identical Assets Level 1 | U.S. Treasury securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	1,291
Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	64,319	68,354
Significant Other Observable Inputs Level 2 | Debt securities of U.S. government-sponsored entities
Assets measured at fair value on a recurring basis:
Total assets at fair value	773	19,350
Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	23,743	19,999
Significant Other Observable Inputs Level 2 | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	39,803	29,005
Fair value
Assets measured at fair value on a recurring basis:
Total assets at fair value	75,770	75,731
Fair value | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	10,160	7,377
Fair value | U.S. Treasury securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	1,291
Fair value | Debt securities of U.S. government-sponsored entities
Assets measured at fair value on a recurring basis:
Total assets at fair value	773	19,350
Fair value | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	23,743	19,999
Fair value | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 39,803	$ 29,005