General (Details 5) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Short-term and long-term investments
Amortized Cost	$ 189,693		$ 201,149
Gross Unrealized Gains	65		34
Gross Unrealized Losses	(166)		(280)
Total, Estimated Fair Value	189,592		200,903
Amortized Cost
Maturity dates, investments, 2012	116,533
Maturity dates, investments, 2013	68,078
Maturity dates, investments, 2014	5,082
Total, Amortized Cost	189,693
Estimated Fair Value
Maturity dates, investments, 2012	116,517
Maturity dates, investments, 2013	68,020
Maturity dates, investments, 2014	5,055
Total, Estimated Fair Value	189,592		200,903
Income taxes
Effective income tax rates (as a percent)	41.00%	40.50%
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	536		697
Total, Estimated Fair Value	536		697
Estimated Fair Value
Total, Estimated Fair Value	536		697
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	8,127		8,293
Gross Unrealized Gains	3		3
Gross Unrealized Losses			(3)
Total, Estimated Fair Value	8,130		8,293
Estimated Fair Value
Total, Estimated Fair Value	8,130		8,293
Corporate debt securities
Short-term and long-term investments
Amortized Cost	171,313		192,059
Gross Unrealized Gains	62		31
Gross Unrealized Losses	(161)		(277)
Total, Estimated Fair Value	171,214		191,813
Estimated Fair Value
Total, Estimated Fair Value	171,214		191,813
Certificates of deposit
Short-term and long-term investments
Amortized Cost	150		100
Total, Estimated Fair Value	150		100
Estimated Fair Value
Total, Estimated Fair Value	150		100
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost	9,567
Gross Unrealized Losses	(5)
Total, Estimated Fair Value	9,562
Estimated Fair Value
Total, Estimated Fair Value	$ 9,562