Marketable Securities (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Marketable securities, Amortized Cost Basis	$ 20,133		$ 20,133		$ 94,433
Marketable securities, Gross Unrealized Holding Gains	9		9		62
Marketable securities, Gross Unrealized Holding Losses	(1)		(1)		(35)
Marketable securities, Aggregate Fair Value	20,141		20,141		94,460
Marketable securities, realized gains and losses	0	0	0	0
Commercial Paper [Member]
Marketable securities, Amortized Cost Basis	4,998		4,998		65,244
Marketable securities, Gross Unrealized Holding Gains					44
Marketable securities, Gross Unrealized Holding Losses					(27)
Marketable securities, Aggregate Fair Value	4,998		4,998		65,261
Corporate Bonds [Member]
Marketable securities, Amortized Cost Basis	15,135		15,135		19,186
Marketable securities, Gross Unrealized Holding Gains	9		9		6
Marketable securities, Gross Unrealized Holding Losses	(1)		(1)		(8)
Marketable securities, Aggregate Fair Value	15,143		15,143		19,184
Federal Agency Obligations [Member]
Marketable securities, Amortized Cost Basis					10,003
Marketable securities, Gross Unrealized Holding Gains					12
Marketable securities, Aggregate Fair Value					$ 10,015