Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 security	Dec. 31, 2012
Investments
Cost Basis, Securities included in deposits and other assets	$ 625	$ 625
Estimated Fair Value, Securities included in deposits and other assets	625	625
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	145
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	243,690
Less than 12 months of temporary impairment, Unrealized Losses	(601)
Available-for-sale securities:
Investments
Amortized Cost	416,430	253,839
Unrealized Gains	261	310
Unrealized Losses	(601)	(185)
Estimated Fair Value	416,090	253,964
Available-for-sale securities: | Debt maturities of one year or less
Investments
Amortized Cost	162,180	144,909
Unrealized Gains	105	102
Unrealized Losses	(87)	(77)
Estimated Fair Value	162,198	144,934
Available-for-sale securities: | Debt maturities of more than one year
Investments
Amortized Cost	254,250	108,930
Unrealized Gains	156	208
Unrealized Losses	(514)	(108)
Estimated Fair Value	253,892	109,030
Corporate debt securities
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	137
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	218,610
Less than 12 months of temporary impairment, Unrealized Losses	(551)
Corporate debt securities | Debt maturities of one year or less
Investments
Amortized Cost	119,472	115,249
Unrealized Gains	65	81
Unrealized Losses	(44)	(9)
Estimated Fair Value	119,493	115,321
Corporate debt securities | Debt maturities of more than one year
Investments
Amortized Cost	220,494	80,166
Unrealized Gains	87	112
Unrealized Losses	(507)	(92)
Estimated Fair Value	220,074	80,186
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	19,976
Less than 12 months of temporary impairment, Unrealized Losses	(33)
Debt securities issued by U.S. government agencies | Debt maturities of one year or less
Investments
Amortized Cost	19,341	12,100
Unrealized Gains	11	2
Unrealized Losses	(27)	(66)
Estimated Fair Value	19,325	12,036
Debt securities issued by U.S. government agencies | Debt maturities of more than one year
Investments
Amortized Cost	17,479	8,034
Unrealized Gains	22	38
Unrealized Losses	(6)
Estimated Fair Value	17,495	8,072
Debt securities issued by the U.S. Treasury | Debt maturities of one year or less
Investments
Amortized Cost	7,257	1,000
Unrealized Gains	10	1
Estimated Fair Value	7,267	1,001
Debt securities issued by the U.S. Treasury | Debt maturities of more than one year
Investments
Amortized Cost	9,075	12,424
Unrealized Gains	22	27
Estimated Fair Value	9,097	12,451
Debt securities issued by states of the United States and political subdivisions of the states
Temporarily impaired investments
Temporarily impaired securities, Number of Investments	4
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated Fair Value	5,104
Less than 12 months of temporary impairment, Unrealized Losses	(17)
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	16,110	16,560
Unrealized Gains	19	18
Unrealized Losses	(16)	(2)
Estimated Fair Value	16,113	16,576
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	7,202	8,306
Unrealized Gains	25	31
Unrealized Losses	(1)	(16)
Estimated Fair Value	7,226	8,321
Equity securities
Investments
Cost Basis	17,689	17,730
Unrealized Gains	50,652	22,271
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	67,461	39,121
Equity securities | Securities included in other current assets
Investments
Cost Basis	1,538	1,579
Unrealized Gains	1,174	4,175
Other-Than-Temporary Impairment Loss	(880)	(880)
Estimated Fair Value	1,832	4,874
Equity securities | Regulus Therapeutics Inc.
Investments
Cost Basis	15,526	15,526
Unrealized Gains	49,478	18,096
Estimated Fair Value	$ 65,004	$ 33,622