Investments, Summary of Investments and Investments Temporarily Impaired (Details) $ in Thousands		Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Summary of Investments [Abstract]
Cost	[1]	$ 660,458	$ 606,889
Gross unrealized gains	[1]	30	172
Gross unrealized losses	[1]	(2,552)	(1,154)
Estimated fair value	[1]	657,936	605,907
Cost		667,620	619,366
Gross unrealized gains		17,660	69,576
Gross unrealized losses		(2,552)	(1,154)
Estimated fair value		$ 682,728	687,788
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		648
Estimated fair value, less than 12 months of temporary impairment		$ 540,423
Unrealized losses, less than 12 months of temporary impairment		(2,346)
Estimated fair value, more than 12 months of temporary impairment		46,701
Unrealized losses, more than 12 months of temporary impairment		(206)
Estimated fair value, total temporary impairment		587,124
Unrealized losses, total temporary impairment		(2,552)
Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	314,247	331,556
Gross unrealized gains	[1]	24	124
Gross unrealized losses	[1]	(360)	(169)
Estimated fair value	[1]	313,911	331,511
Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	346,211	275,333
Gross unrealized gains	[1]	6	48
Gross unrealized losses	[1]	(2,192)	(985)
Estimated fair value	[1]	$ 344,025	274,396
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		397
Estimated fair value, less than 12 months of temporary impairment		$ 406,098
Unrealized losses, less than 12 months of temporary impairment		(1,908)
Estimated fair value, more than 12 months of temporary impairment		16,066
Unrealized losses, more than 12 months of temporary impairment		(47)
Estimated fair value, total temporary impairment		422,164
Unrealized losses, total temporary impairment		(1,955)
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	181,670	219,856	[2]
Gross unrealized gains	[1]	5	89	[2]
Gross unrealized losses	[1]	(250)	(89)	[2]
Estimated fair value	[1]	181,425	219,856	[2]
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	258,703	152,730
Gross unrealized gains	[1]	3	16
Gross unrealized losses	[1]	(1,705)	(600)
Estimated fair value	[1]	$ 257,001	152,146
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		19
Estimated fair value, less than 12 months of temporary impairment		$ 71,842
Unrealized losses, less than 12 months of temporary impairment		(262)
Estimated fair value, more than 12 months of temporary impairment		1,001
Unrealized losses, more than 12 months of temporary impairment		(1)
Estimated fair value, total temporary impairment		72,843
Unrealized losses, total temporary impairment		(263)
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	50,559	47,496
Gross unrealized gains	[1]	1	7
Gross unrealized losses	[1]	(19)	(27)
Estimated fair value	[1]	50,541	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]	38,956	62,530
Gross unrealized gains	[1]	0	0
Gross unrealized losses	[1]	(244)	(151)
Estimated fair value	[1]	$ 38,712	62,379
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		2
Estimated fair value, less than 12 months of temporary impairment		$ 2,601
Unrealized losses, less than 12 months of temporary impairment		(3)
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		2,601
Unrealized losses, total temporary impairment		(3)
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	2,604	19,008	[2]
Gross unrealized gains	[1]	0	9	[2]
Gross unrealized losses	[1]	(3)	0	[2]
Estimated fair value	[1]	$ 2,601	19,017	[2]
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		230
Estimated fair value, less than 12 months of temporary impairment		$ 59,882
Unrealized losses, less than 12 months of temporary impairment		(173)
Estimated fair value, more than 12 months of temporary impairment		29,634
Unrealized losses, more than 12 months of temporary impairment		(158)
Estimated fair value, total temporary impairment		89,516
Unrealized losses, total temporary impairment		(331)
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]	79,414	45,196
Gross unrealized gains	[1],[2]	18	19
Gross unrealized losses	[1],[2]	(88)	(53)
Estimated fair value	[1],[2]	79,344	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]	48,552	60,073
Gross unrealized gains	[1]	3	32
Gross unrealized losses	[1]	(243)	(234)
Estimated fair value	[1]	48,312	59,871
Equity Securities [Member]
Summary of Investments [Abstract]
Cost		7,162	12,477
Gross unrealized gains		17,630	69,404
Gross unrealized losses		0	0
Estimated fair value		24,792	81,881
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Summary of Investments [Abstract]
Cost		7,162	12,477
Gross unrealized gains		17,630	69,404
Gross unrealized losses		0	0
Estimated fair value		$ 24,792	$ 81,881

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

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