Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014 Investment	Dec. 31, 2013
Investments
Cost Basis	$ 496,158	$ 527,873
Unrealized Gains	50,728	37,535
Unrealized Losses	(446)	(619)
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	545,560	563,909
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	150
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	207,144
Less than 12 months of temporary impairment, Unrealized Losses	(444)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated Fair Value	6,057
More than 12 months of temporary impairment, Unrealized Losses	(2)
Total temporary impairment, Estimated Fair Value	213,201
Total temporary impairment, Unrealized Losses	(446)
Available-for-sale securities [Member]
Investments
Amortized Cost	478,581	510,184
Unrealized Gains	311	347
Unrealized Losses	(446)	(619)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	478,446	509,912
Available-for-sale securities [Member] | Debt maturities of one year or less [Member]
Investments
Amortized Cost	227,784	179,659
Unrealized Gains	144	109
Unrealized Losses	(61)	(71)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	227,867	179,697
Available-for-sale securities [Member] | Debt maturities of more than one year [Member]
Investments
Amortized Cost	250,797	330,525
Unrealized Gains	167	238
Unrealized Losses	(385)	(548)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	250,579	330,215
Corporate debt securities [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	124
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	144,662
Less than 12 months of temporary impairment, Unrealized Losses	(207)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated Fair Value	6,057
More than 12 months of temporary impairment, Unrealized Losses	(2)
Total temporary impairment, Estimated Fair Value	150,719
Total temporary impairment, Unrealized Losses	(209)
Corporate debt securities [Member] | Debt maturities of one year or less [Member]
Investments
Amortized Cost	176,874	142,096
Unrealized Gains	104	75
Unrealized Losses	(32)	(27)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	176,946	142,144
Corporate debt securities [Member] | Debt maturities of more than one year [Member]
Investments
Amortized Cost	173,937	265,969
Unrealized Gains	101	177
Unrealized Losses	(177)	(393)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	173,861	265,753
Debt securities issued by U.S. government agencies [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	7
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	47,622
Less than 12 months of temporary impairment, Unrealized Losses	(153)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated Fair Value	0
More than 12 months of temporary impairment, Unrealized Losses	0
Total temporary impairment, Estimated Fair Value	47,622
Total temporary impairment, Unrealized Losses	(153)
Debt securities issued by U.S. government agencies [Member] | Debt maturities of one year or less [Member]
Investments
Amortized Cost	18,427	23,242
Unrealized Gains	17	22
Unrealized Losses	0	(16)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	18,444	23,248
Debt securities issued by U.S. government agencies [Member] | Debt maturities of more than one year [Member]
Investments
Amortized Cost	49,303	41,308
Unrealized Gains	4	3
Unrealized Losses	(153)	(127)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	49,154	41,184
Debt securities issued by the U.S. Treasury [Member] | Debt maturities of one year or less [Member]
Investments
Amortized Cost	9,272	6,239
Unrealized Gains	14	6
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	9,286	6,245
Debt securities issued by the U.S. Treasury [Member] | Debt maturities of more than one year [Member]
Investments
Amortized Cost	5,998	9,062
Unrealized Gains	14	21
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	6,012	9,083
Debt securities issued by states of the United States and political subdivisions of the states [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	19
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	14,860
Less than 12 months of temporary impairment, Unrealized Losses	(84)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated Fair Value	0
More than 12 months of temporary impairment, Unrealized Losses	0
Total temporary impairment, Estimated Fair Value	14,860
Total temporary impairment, Unrealized Losses	(84)
Debt securities issued by states of the United States and political subdivisions of the states [Member] | Debt maturities of one year or less [Member]
Investments
Amortized Cost	23,211	8,082
Unrealized Gains	9	6
Unrealized Losses	(29)	(28)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	23,191	8,060
Debt securities issued by states of the United States and political subdivisions of the states [Member] | Debt maturities of more than one year [Member]
Investments
Amortized Cost	21,559	14,186
Unrealized Gains	48	37
Unrealized Losses	(55)	(28)
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	21,552	14,195
Equity securities [Member]
Investments
Cost Basis	17,577	17,689
Unrealized Gains	50,417	37,188
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	67,114	53,997
Equity securities [Member] | Securities included in other current assets [Member]
Investments
Cost Basis	1,426	1,538
Unrealized Gains	2,357	618
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	2,903	1,276
Equity securities [Member] | Securities included in deposits and other assets [Member]
Investments
Cost Basis, Securities included in deposits and other assets	625	625
Estimated Fair Value, Securities included in deposits and other assets	625	625
Unrealized Gains	0	0
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	0	0
Equity securities [Member] | Regulus Therapeutics Inc. [Member]
Investments
Cost Basis	15,526	15,526
Unrealized Gains	48,060	36,570
Unrealized Losses	0	0
Other-Than-Temporary Impairment Loss	0	0
Estimated Fair Value	$ 63,586	$ 52,096