Investments, Summary of Investments and Investments Temporarily Impaired (Details) $ in Thousands		Mar. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)
Summary of Investments [Abstract]
Cost		$ 623,181	$ 667,620
Gross unrealized gains		13,226	17,660
Gross unrealized losses		(670)	(2,552)
Estimated fair value		$ 635,737	682,728
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		381
Estimated fair value, less than 12 months of temporary impairment		$ 308,877
Unrealized losses, less than 12 months of temporary impairment		(550)
Estimated fair value, more than 12 months of temporary impairment		35,976
Unrealized losses, more than 12 months of temporary impairment		(120)
Estimated fair value, total temporary impairment		344,853
Unrealized losses, total temporary impairment		(670)
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	616,019	660,458
Gross unrealized gains		685	30
Gross unrealized losses		(670)	(2,552)
Estimated fair value		616,034	657,936
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	295,872	314,247
Gross unrealized gains		49	24
Gross unrealized losses		(160)	(360)
Estimated fair value		295,761	313,911
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	320,147	346,211
Gross unrealized gains		636	6
Gross unrealized losses		(510)	(2,192)
Estimated fair value		$ 320,273	344,025
Corporate Debt Securities [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		168
Estimated fair value, less than 12 months of temporary impairment		$ 203,743
Unrealized losses, less than 12 months of temporary impairment		(442)
Estimated fair value, more than 12 months of temporary impairment		16,040
Unrealized losses, more than 12 months of temporary impairment		(33)
Estimated fair value, total temporary impairment		219,783
Unrealized losses, total temporary impairment		(475)
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	161,995	181,670	[2]
Gross unrealized gains		29	5
Gross unrealized losses		(59)	(250)
Estimated fair value		161,965	181,425
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	242,995	258,703
Gross unrealized gains		608	3
Gross unrealized losses		(416)	(1,705)
Estimated fair value		$ 243,187	257,001
Debt Securities issued by U.S. Government Agencies [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		8
Estimated fair value, less than 12 months of temporary impairment		$ 28,890
Unrealized losses, less than 12 months of temporary impairment		(13)
Estimated fair value, more than 12 months of temporary impairment		1,000
Unrealized losses, more than 12 months of temporary impairment		0
Estimated fair value, total temporary impairment		29,890
Unrealized losses, total temporary impairment		(13)
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1],[2]	42,545	50,559
Gross unrealized gains		3	1
Gross unrealized losses		(1)	(19)
Estimated fair value		42,547	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]	36,951	38,956
Gross unrealized gains		2	0
Gross unrealized losses		(12)	(244)
Estimated fair value		$ 36,941	38,712
Debt Securities issued by the U.S. Treasury [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		1
Estimated fair value, less than 12 months of temporary impairment		$ 10,523
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		10,523
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]	13,101	2,604
Gross unrealized gains		3	0
Gross unrealized losses		(3)	(3)
Estimated fair value		$ 13,101	2,601
Debt Securities issued by States of the United States and Political Subdivisions of the States [Member]
Temporarily Impaired Investments [Abstract]
Number of investments | Investment		204
Estimated fair value, less than 12 months of temporary impairment		$ 65,721
Unrealized losses, less than 12 months of temporary impairment		(92)
Estimated fair value, more than 12 months of temporary impairment		18,936
Unrealized losses, more than 12 months of temporary impairment		(87)
Estimated fair value, total temporary impairment		84,657
Unrealized losses, total temporary impairment		(179)
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]	78,231	79,414
Gross unrealized gains		14	18
Gross unrealized losses		(97)	(88)
Estimated fair value		78,148	79,344
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]	40,201	48,552
Gross unrealized gains		26	3
Gross unrealized losses		(82)	(243)
Estimated fair value		40,145	48,312
Equity Securities [Member]
Summary of Investments [Abstract]
Cost		7,162	7,162
Gross unrealized gains		12,541	17,630
Gross unrealized losses		0	0
Estimated fair value		19,703	24,792
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Summary of Investments [Abstract]
Cost		7,162	7,162
Gross unrealized gains		12,541	17,630
Gross unrealized losses		0	0
Estimated fair value		$ 19,703	$ 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.