Investments (Details 2) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Investments
Amortized cost, long-term equity securities	$ 625	$ 625
Unrealized Gains	1,323	1,884
Unrealized Losses	(119)	(130)
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value, Long-term Equity Securities	625	625
Amortized Cost, Investments	312,060	404,063
Estimated Fair Value, Investments	312,384	404,937
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	52
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	112,644
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(119)
Short-term investments
Investments
Amortized Cost, Debt Securities	309,897	401,900
Unrealized Gains	384	531
Unrealized Losses	(119)	(130)
Estimated Fair Value	310,162	402,301
Short-term investments \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	238,459	346,919
Unrealized Gains	231	360
Unrealized Losses	(59)	(75)
Estimated Fair Value	238,631	347,204
Short-term investments \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	71,438	54,981
Unrealized Gains	153	171
Unrealized Losses	(60)	(55)
Estimated Fair Value	71,531	55,097
Corporate debt securities
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	45
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	97,370
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(108)
Corporate debt securities \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	167,417	196,010
Unrealized Gains	196	294
Unrealized Losses	(51)	(41)
Estimated Fair Value	167,562	196,263
Corporate debt securities \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	57,591	47,842
Unrealized Gains	140	167
Unrealized Losses	(57)	(44)
Estimated Fair Value	57,674	47,965
Debt securities issued by U.S. government agencies
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	4
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	11,541
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(8)
Debt securities issued by U.S. government agencies \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	57,128	119,890
Unrealized Gains	25	53
Unrealized Losses	(8)	(34)
Estimated Fair Value	57,145	119,909
Debt securities issued by U.S. government agencies \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	10,117	7,139
Unrealized Gains	13	4
Unrealized Losses		(11)
Estimated Fair Value	10,130	7,132
Debt securities issued by the U.S. Treasury \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	6,513	24,030
Unrealized Gains	10	10
Estimated Fair Value	6,523	24,040
Debt securities issued by states of the United States and political subdivisions of the states
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	3
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	3,733
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(3)
Debt securities issued by states of the United States and political subdivisions of the states \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	7,401	6,989
Unrealized Gains		3
Estimated Fair Value	7,401	6,992
Debt securities issued by states of the United States and political subdivisions of the states \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	3,730
Unrealized Losses	(3)
Estimated Fair Value	3,727
Equity securities
Investments
Unrealized Gains	939	1,353
Other-Than-Temporary Impairment Loss	(880)	(880)
Amortized Cost, Investments	2,163	2,163
Estimated Fair Value, Investments	2,222	2,636
Current portion of equity securities
Investments
Amortized Cost, Equity Securities	1,538	1,538
Unrealized Gains	939	1,353
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	$ 1,597	$ 2,011

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