General (Details 6) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Short-term and long-term investments
Amortized Cost	$ 254,627,000	$ 254,627,000		$ 284,489,000
Gross Unrealized Gains	57,000	57,000		253,000
Gross Unrealized Losses	(535,000)	(535,000)		(238,000)
Estimated Fair Value	254,149,000	254,149,000		284,504,000
Amortized Cost
Maturity dates, investments, 2011	92,150,000	92,150,000
Maturity dates, investments, 2012	159,009,000	159,009,000
Maturity dates, investments, 2013	3,468,000	3,468,000
Total, Amortized Cost	254,627,000	254,627,000
Estimated Fair Value
Maturity dates, investments, 2011	92,062,000	92,062,000
Maturity dates, investments, 2012	158,644,000	158,644,000
Maturity dates, investments, 2013	3,443,000	3,443,000
Total, Estimated Fair Value	254,149,000	254,149,000		284,504,000
Income taxes
Effective income tax rates (as a percent)		34.00%	40.30%
The ownership percentage for which the entity files a consolidated federal income tax return, low end of range	80.00%	80.00%
Unrecognized tax benefit, excluding interest, reversed in the reporting period due to expiration of statute of limitations	9,600,000
Unrecognized tax benefits that would impact effective tax rate	8,100,000	8,100,000
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	698,000	698,000		2,178,000
Gross Unrealized Gains				1,000
Estimated Fair Value	698,000	698,000		2,179,000
Estimated Fair Value
Total, Estimated Fair Value	698,000	698,000		2,179,000
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	11,160,000	11,160,000		10,142,000
Gross Unrealized Gains	4,000	4,000		7,000
Gross Unrealized Losses	(3,000)	(3,000)		(11,000)
Estimated Fair Value	11,161,000	11,161,000		10,138,000
Estimated Fair Value
Total, Estimated Fair Value	11,161,000	11,161,000		10,138,000
Corporate debt securities
Short-term and long-term investments
Amortized Cost	242,569,000	242,569,000		268,739,000
Gross Unrealized Gains	53,000	53,000		245,000
Gross Unrealized Losses	(532,000)	(532,000)		(215,000)
Estimated Fair Value	242,090,000	242,090,000		268,769,000
Estimated Fair Value
Total, Estimated Fair Value	242,090,000	242,090,000		268,769,000
Certificates of deposit
Short-term and long-term investments
Amortized Cost	200,000	200,000		750,000
Estimated Fair Value	200,000	200,000		750,000
Estimated Fair Value
Total, Estimated Fair Value	200,000	200,000		750,000
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost				2,680,000
Gross Unrealized Losses				(12,000)
Estimated Fair Value				2,668,000
Estimated Fair Value
Total, Estimated Fair Value				$ 2,668,000

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