Investments, Summary of Investments and Investments Temporarily Impaired (Details) $ in Thousands		Jun. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Summary of investments [Abstract]
Cost		$ 668,611	$ 620,246
Gross unrealized gains		48,285	69,576
Gross unrealized losses		(1,199)	(1,154)
Other-than-temporary impairment loss		(880)	(880)
Estimated fair value		$ 714,817	687,788
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments | Investment		315
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value		$ 410,225
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses		(1,086)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value		15,492
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses		(113)
Total temporary impairment, Estimated fair value		425,717
Total temporary impairment, Unrealized losses		(1,199)
Available-for-sale securities [Member]
Summary of investments [Abstract]
Cost	[1]	655,254	606,889
Gross unrealized gains		158	172
Gross unrealized losses		(1,199)	(1,154)
Other-than-temporary impairment loss		0	0
Estimated fair value		654,213	605,907
Available-for-sale securities [Member] | Debt maturities of one year or less [Member]
Summary of investments [Abstract]
Cost	[1]	330,961	331,556
Gross unrealized gains		64	124
Gross unrealized losses		(159)	(169)
Other-than-temporary impairment loss		0	0
Estimated fair value		330,866	331,511
Available-for-sale securities [Member] | Debt maturities of more than one year [Member]
Summary of investments [Abstract]
Cost	[1]	324,293	275,333
Gross unrealized gains		94	48
Gross unrealized losses		(1,040)	(985)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 323,347	274,396
Corporate Debt Securities [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments | Investment		254
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value		$ 296,493
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses		(894)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value		9,263
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses		(44)
Total temporary impairment, Estimated fair value		305,756
Total temporary impairment, Unrealized losses		(938)
Corporate Debt Securities [Member] | Debt maturities of one year or less [Member]
Summary of investments [Abstract]
Cost	[1]	189,487	219,856	[2]
Gross unrealized gains		37	89	[2]
Gross unrealized losses		(90)	(89)	[2]
Other-than-temporary impairment loss		0	0	[2]
Estimated fair value		189,434	219,856	[2]
Corporate Debt Securities [Member] | Debt maturities of more than one year [Member]
Summary of investments [Abstract]
Cost	[1]	234,088	152,730
Gross unrealized gains		52	16
Gross unrealized losses		(848)	(600)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 233,292	152,146
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily impaired investments [Abstract]
Temporarily impaired securities, Number of Investments | Investment		12
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value		$ 62,116
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses		(14)
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		62,116
Total temporary impairment, Unrealized losses		(14)
Debt Securities issued by U.S. Government Agencies [Member] | Debt maturities of one year or less [Member]
Summary of investments [Abstract]
Cost	[1]	81,639	47,496
Gross unrealized gains		5	7
Gross unrealized losses		(11)	(27)
Other-than-temporary impairment loss		0	0
Estimated fair value		81,633	47,476
Debt Securities issued by U.S. Government Agencies [Member] | Debt maturities of more than one year [Member]
Summary of investments [Abstract]
Cost	[1]	28,002	62,530
Gross unrealized gains		10	0
Gross unrealized losses		(3)	(151)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 28,009	62,379
Debt Securities issued by the U.S. Treasury [Member] | Debt maturities of one year or less [Member]
Summary of investments [Abstract]
Cost	[1],[2]		19,008
Gross unrealized gains	[2]		9
Gross unrealized losses	[2]		0
Other-than-temporary impairment loss	[2]		0
Estimated fair value	[2]		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 | Investment		49
Temporarily impaired securities, Less than 12 months of temporary impairment, Estimated fair value		$ 51,616
Temporarily impaired securities, Less than 12 months of temporary impairment, Unrealized losses		(178)
Temporarily impaired securities, More than 12 months of temporary impairment, Estimated fair value		6,229
Temporarily impaired securities, More than 12 months of temporary impairment, Unrealized losses		(69)
Total temporary impairment, Estimated fair value		57,845
Total temporary impairment, Unrealized losses		(247)
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Debt maturities of one year or less [Member]
Summary of investments [Abstract]
Cost	[1],[2]	59,835	45,196
Gross unrealized gains	[2]	22	19
Gross unrealized losses	[2]	(58)	(53)
Other-than-temporary impairment loss	[2]	0	0
Estimated fair value	[2]	59,799	45,162
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Debt maturities of more than one year [Member]
Summary of investments [Abstract]
Cost	[1]	62,203	60,073
Gross unrealized gains		32	32
Gross unrealized losses		(189)	(234)
Other-than-temporary impairment loss		0	0
Estimated fair value		62,046	59,871
Equity Securities [Member]
Summary of investments [Abstract]
Cost		13,357	13,357
Gross unrealized gains		48,127	69,404
Gross unrealized losses		0	0
Other-than-temporary impairment loss		(880)	(880)
Estimated fair value		60,604	81,881
Equity Securities [Member] | Securities included in other current assets [Member]
Summary of investments [Abstract]
Cost		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]
Summary of investments [Abstract]
Cost		12,477	12,477
Gross unrealized gains		48,127	69,404
Gross unrealized losses		0	0
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 60,604	$ 81,881

[1] Our available-for-sale securities are held at amortized cost.

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