Financial Instruments and Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of assets measured at fair value on a recurring basis
Assets at fair value	$ 172,381	$ 138,285
Money market [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value	172,381	136,273
U.S. agency obligations [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value		2,012
Level 1 [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value	172,381	136,273
Level 1 [Member] | Money market [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value	172,381	136,273
Level 1 [Member] | U.S. agency obligations [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value
Level 2 [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value		2,012
Level 2 [Member] | Money market [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value
Level 2 [Member] | U.S. agency obligations [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value		2,012
Level 3 [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value
Level 3 [Member] | Money market [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value
Level 3 [Member] | U.S. agency obligations [Member]
Schedule of assets measured at fair value on a recurring basis
Assets at fair value