Investments, Summary of Investments and Investments Temporarily Impaired (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 Investment		Dec. 31, 2014
Investment Holdings [Line Items]
Cost basis	$ 569,220		$ 620,246
Gross unrealized gains	81,291		69,576
Gross unrealized losses	(435)		(1,154)
Other-than-temporary impairment loss	(880)		(880)
Estimated fair value	649,195		687,788
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	167
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value	253,640
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses	(364)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value	6,231
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses	(71)
Total temporary impairment, Estimated fair value	259,871
Total temporary impairment, Unrealized losses	(435)
Available-for-sale securities [Member]
Investment Holdings [Line Items]
Amortized cost	555,862		606,889
Gross unrealized gains	322		172
Gross unrealized losses	(435)		(1,154)
Other-than-temporary impairment loss	0		0
Estimated fair value	555,749		605,907
Available-for-sale securities [Member] | Debt maturities of one year or less [Member]
Investment Holdings [Line Items]
Amortized cost	296,014		331,556
Gross unrealized gains	121		124
Gross unrealized losses	(94)		(169)
Other-than-temporary impairment loss	0		0
Estimated fair value	296,040		331,511
Available-for-sale securities [Member] | Debt maturities of more than one year [Member]
Investment Holdings [Line Items]
Amortized cost	259,849		275,333
Gross unrealized gains	201		48
Gross unrealized losses	(341)		(985)
Other-than-temporary impairment loss	0		0
Estimated fair value	259,709		274,396
Corporate Debt Securities [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	132
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value	136,603
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses	(193)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value	0
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses	0
Total temporary impairment, Estimated fair value	136,603
Total temporary impairment, Unrealized losses	(193)
Corporate Debt Securities [Member] | Debt maturities of one year or less [Member]
Investment Holdings [Line Items]
Amortized cost	190,437	[1]	219,856	[1]
Gross unrealized gains	87	[1]	89	[1]
Gross unrealized losses	(31)	[1]	(89)	[1]
Other-than-temporary impairment loss	0	[1]	0	[1]
Estimated fair value	190,493	[1]	219,856	[1]
Corporate Debt Securities [Member] | Debt maturities of more than one year [Member]
Investment Holdings [Line Items]
Amortized cost	137,653		152,730
Gross unrealized gains	124		16
Gross unrealized losses	(162)		(600)
Other-than-temporary impairment loss	0		0
Estimated fair value	137,615		152,146
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments	11
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value	74,094
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses	(27)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value	0
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses	0
Total temporary impairment, Estimated fair value	74,094
Total temporary impairment, Unrealized losses	(27)
Debt Securities issued by U.S. Government Agencies [Member] | Debt maturities of one year or less [Member]
Investment Holdings [Line Items]
Amortized cost	53,006	[1]	47,496	[1]
Gross unrealized gains	5	[1]	7	[1]
Gross unrealized losses	(13)	[1]	(27)	[1]
Other-than-temporary impairment loss	0	[1]	0	[1]
Estimated fair value	52,998	[1]	47,476	[1]
Debt Securities issued by U.S. Government Agencies [Member] | Debt maturities of more than one year [Member]
Investment Holdings [Line Items]
Amortized cost	62,529		62,530
Gross unrealized gains	29		0
Gross unrealized losses	(14)		(151)
Other-than-temporary impairment loss	0		0
Estimated fair value	62,544		62,379
Debt Securities issued by the U.S. Treasury [Member] | Debt maturities of one year or less [Member]
Investment Holdings [Line Items]
Amortized cost	5,999		19,008
Gross unrealized gains	4		9
Gross unrealized losses	0		0
Other-than-temporary impairment loss	0		0
Estimated fair value	6,003		19,017
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	24
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value	42,943
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses	(144)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value	6,231
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses	(71)
Total temporary impairment, Estimated fair value	49,174
Total temporary impairment, Unrealized losses	(215)
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Debt maturities of one year or less [Member]
Investment Holdings [Line Items]
Amortized cost	46,571	[1]	45,196	[1]
Gross unrealized gains	25	[1]	19	[1]
Gross unrealized losses	(50)	[1]	(53)	[1]
Other-than-temporary impairment loss	0	[1]	0	[1]
Estimated fair value	46,546	[1]	45,162	[1]
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Debt maturities of more than one year [Member]
Investment Holdings [Line Items]
Amortized cost	59,667		60,073
Gross unrealized gains	48		32
Gross unrealized losses	(165)		(234)
Other-than-temporary impairment loss	0		0
Estimated fair value	59,550		59,871
Equity Securities [Member]
Investment Holdings [Line Items]
Cost basis	13,357		13,357
Gross unrealized gains	80,969		69,404
Gross unrealized losses	0		0
Other-than-temporary impairment loss	(880)		(880)
Estimated fair value	93,446		81,881
Equity Securities [Member] | Securities included in other current assets [Member]
Investment Holdings [Line Items]
Cost basis	880		880
Gross unrealized gains	0		0
Gross unrealized losses	0		0
Other-than-temporary impairment loss	(880)		(880)
Estimated fair value	0		0
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Investment Holdings [Line Items]
Cost basis	12,477		12,477
Gross unrealized gains	80,969		69,404
Gross unrealized losses	0		0
Other-than-temporary impairment loss	0		0
Estimated fair value	$ 93,446		$ 81,881

[1] Includes investments classified as cash equivalents on our condensed consolidated balance sheet.