Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Dec. 31, 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	$ 8,679	$ 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	7,403	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	1,276
Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	86,751	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	761	19,350
Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	33,888	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	52,102	29,005
Fair value
Assets measured at fair value on a recurring basis:
Total assets at fair value	95,430	75,731
Fair value | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	7,403	7,377
Fair value | U.S. Treasury securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	1,276
Fair value | Debt securities of U.S. government-sponsored entities
Assets measured at fair value on a recurring basis:
Total assets at fair value	761	19,350
Fair value | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	33,888	19,999
Fair value | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 52,102	$ 29,005