Investments (Details 2) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2011 Y D entity count	Dec. 31, 2010
Investments
Amortized cost, long-term equity securities	$ 625	$ 625
Unrealized Gains	612	1,884
Unrealized Losses	(960)	(130)
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value, Long-term Equity Securities	625	625
Amortized Cost, Investments	312,522	404,063
Estimated Fair Value, Investments	311,294	404,937
Temporarily impaired investments
Temporarily impaired securities, Number of Investments (in counts)	77
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	168,175
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(960)
Short-term investments
Investments
Amortized Cost, Debt Securities	310,359	401,900
Unrealized Gains	101	531
Unrealized Losses	(960)	(130)
Estimated Fair Value	309,500	402,301
Short-term investments \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	234,960	346,919
Unrealized Gains	59	360
Unrealized Losses	(389)	(75)
Estimated Fair Value	234,630	347,204
Short-term investments \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	75,399	54,981
Unrealized Gains	42	171
Unrealized Losses	(571)	(55)
Estimated Fair Value	74,870	55,097
Corporate debt securities
Temporarily impaired investments
Temporarily impaired securities, Number of Investments (in counts)	66
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	137,205
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(792)
Corporate debt securities \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	149,296	196,010
Unrealized Gains	28	294
Unrealized Losses	(380)	(41)
Estimated Fair Value	148,944	196,263
Corporate debt securities \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	59,430	47,842
Unrealized Gains	35	167
Unrealized Losses	(412)	(44)
Estimated Fair Value	59,053	47,965
Debt securities issued by U.S. government agencies
Temporarily impaired investments
Temporarily impaired securities, Number of Investments (in counts)	9
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	28,565
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(166)
Debt securities issued by U.S. government agencies \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	65,707	119,890
Unrealized Gains	22	53
Unrealized Losses	(9)	(34)
Estimated Fair Value	65,720	119,909
Debt securities issued by U.S. government agencies \| Debt maturities of more than one year
Investments
Amortized Cost, Debt Securities	12,263	7,139
Unrealized Gains	6	4
Unrealized Losses	(157)	(11)
Estimated Fair Value	12,112	7,132
Debt securities issued by the U.S. Treasury \| Debt maturities of one year or less
Investments
Amortized Cost, Debt Securities	4,506	24,030
Unrealized Gains	6	10
Estimated Fair Value	4,512	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 (in counts)	2
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	2,405
Total temporarily impaired securities, less than 12 months of temporary impairment, Unrealized Losses	(2)
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	15,451	6,989
Unrealized Gains	3	3
Estimated Fair Value	15,454	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	3,706
Unrealized Gains	1
Unrealized Losses	(2)
Estimated Fair Value	3,705
Equity securities
Investments
Unrealized Gains	511	1,353
Other-Than-Temporary Impairment Loss	(880)	(880)
Amortized Cost, Investments	2,163	2,163
Estimated Fair Value, Investments	1,794	2,636
Current portion of equity securities
Investments
Amortized Cost, Equity Securities	1,538	1,538
Unrealized Gains	511	1,353
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	$ 1,169	$ 2,011

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