Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Summary of Investments [Abstract]
Cost		$ 594,095	$ 667,620
Gross unrealized gains		3,020	17,660
Gross unrealized losses		(1,199)	(2,552)
Estimated fair value		595,916	682,728
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	586,933	660,458
Gross unrealized gains		1,132	30
Gross unrealized losses		(366)	(2,552)
Estimated fair value		587,699	657,936
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	324,876	314,247
Gross unrealized gains		90	24
Gross unrealized losses		(174)	(360)
Estimated fair value		324,792	313,911
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	262,057	346,211
Gross unrealized gains		1,042	6
Gross unrealized losses		(192)	(2,192)
Estimated fair value		262,907	344,025
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	179,005	181,670
Gross unrealized gains		52	5
Gross unrealized losses		(82)	(250)
Estimated fair value		178,975	181,425
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	191,442	258,703
Gross unrealized gains		945	3
Gross unrealized losses		(165)	(1,705)
Estimated fair value		192,222	257,001
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	44,765	50,559
Gross unrealized gains		15	1
Gross unrealized losses		0	(19)
Estimated fair value		44,780	50,541
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	31,950	38,956
Gross unrealized gains		5	0
Gross unrealized losses		(1)	(244)
Estimated fair value		31,954	38,712
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	13,063	2,604
Gross unrealized gains		11	0
Gross unrealized losses		0	(3)
Estimated fair value		13,074	2,601
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	8,983
Gross unrealized gains		14
Gross unrealized losses		0
Estimated fair value		8,997
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1],[2]	88,043	79,414
Gross unrealized gains		12	18
Gross unrealized losses		(92)	(88)
Estimated fair value		87,963	79,344
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	29,682	48,552
Gross unrealized gains		78	3
Gross unrealized losses		(26)	(243)
Estimated fair value		29,734	48,312
Equity Securities [Member]
Summary of Investments [Abstract]
Cost		7,162	7,162
Gross unrealized gains		1,888	17,630
Gross unrealized losses		(833)	0
Estimated fair value		8,217	24,792
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Summary of Investments [Abstract]
Cost		7,162	7,162
Gross unrealized gains		1,888	17,630
Gross unrealized losses		(833)	0
Estimated fair value		$ 8,217	$ 24,792

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

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