Investments, Summary of Investments and Investments Temporarily Impaired (Details) $ in Thousands		Sep. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Summary of Investments [Abstract]
Cost		$ 701,958	$ 620,246
Gross unrealized gains		11,692	69,576
Gross unrealized losses		(977)	(1,154)
Other-than-temporary impairment loss		(880)	(880)
Estimated fair value		$ 711,793	687,788
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		350
Estimated fair value, less than 12 months of temporary impairment		$ 384,919
Unrealized losses, less than 12 months of temporary impairment		(853)
Estimated fair value, more than 12 months of temporary impairment		24,320
Unrealized losses, more than 12 months of temporary impairment		(124)
Estimated fair value, total temporary impairment		409,239
Unrealized losses, total temporary impairment		(977)
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	693,916	606,889
Gross unrealized gains		260	172
Gross unrealized losses		(977)	(1,154)
Other-than-temporary impairment loss		0	0
Estimated fair value		693,199	605,907
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	331,829	331,556
Gross unrealized gains		81	124
Gross unrealized losses		(153)	(169)
Other-than-temporary impairment loss		0	0
Estimated fair value		331,757	331,511
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	362,087	275,333
Gross unrealized gains		179	48
Gross unrealized losses		(824)	(985)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 361,442	274,396
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		247
Estimated fair value, less than 12 months of temporary impairment		$ 293,774
Unrealized losses, less than 12 months of temporary impairment		(773)
Estimated fair value, more than 12 months of temporary impairment		9,421
Unrealized losses, more than 12 months of temporary impairment		(16)
Estimated fair value, total temporary impairment		303,195
Unrealized losses, total temporary impairment		(789)
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	178,172	219,856	[2]
Gross unrealized gains		37	89	[2]
Gross unrealized losses		(88)	(89)	[2]
Other-than-temporary impairment loss		0	0	[2]
Estimated fair value		178,121	219,856	[2]
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	270,045	152,730
Gross unrealized gains		132	16
Gross unrealized losses		(701)	(600)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 269,476	152,146
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		7
Estimated fair value, less than 12 months of temporary impairment		$ 40,497
Unrealized losses, less than 12 months of temporary impairment		(8)
Estimated fair value, more than 12 months of temporary impairment		0
Unrealized losses, more than 12 months of temporary impairment		0
Estimated fair value, total temporary impairment		40,497
Unrealized losses, total temporary impairment		(8)
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	91,609	47,496
Gross unrealized gains		8	7
Gross unrealized losses		(6)	(27)
Other-than-temporary impairment loss		0	0
Estimated fair value		91,611	47,476
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	30,654	62,530
Gross unrealized gains		19	0
Gross unrealized losses		(2)	(151)
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 30,671	62,379
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity 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]
Number of investments | Investment		96
Estimated fair value, less than 12 months of temporary impairment		$ 50,648
Unrealized losses, less than 12 months of temporary impairment		(72)
Estimated fair value, more than 12 months of temporary impairment		14,899
Unrealized losses, more than 12 months of temporary impairment		(108)
Estimated fair value, total temporary impairment		65,547
Unrealized losses, total temporary impairment		(180)
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1],[2]	62,048	45,196
Gross unrealized gains	[2]	36	19
Gross unrealized losses	[2]	(59)	(53)
Other-than-temporary impairment loss	[2]	0	0
Estimated fair value	[2]	62,025	45,162
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	61,388	60,073
Gross unrealized gains		28	32
Gross unrealized losses		(121)	(234)
Other-than-temporary impairment loss		0	0
Estimated fair value		61,295	59,871
Equity Securities [Member]
Summary of Investments [Abstract]
Cost		8,042	13,357
Gross unrealized gains		11,432	69,404
Gross unrealized losses		0	0
Other-than-temporary impairment loss		(880)	(880)
Estimated fair value		18,594	81,881
Equity Securities [Member] | 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		7,162	12,477
Gross unrealized gains		11,432	69,404
Gross unrealized losses		0	0
Other-than-temporary impairment loss		0	0
Estimated fair value		$ 18,594	$ 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.