Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Summary of Investments [Abstract]
Cost		$ 593,872	$ 667,620
Gross unrealized gains		348	17,660
Gross unrealized losses		(2,009)	(2,552)
Estimated fair value		592,211	682,728
Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	323,708	314,247
Gross unrealized gains		29	24
Gross unrealized losses		(319)	(360)
Estimated fair value		323,418	313,911
Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	268,031	346,211
Gross unrealized gains		38	6
Gross unrealized losses		(1,690)	(2,192)
Estimated fair value		266,379	344,025
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	591,739	660,458
Gross unrealized gains		67	30
Gross unrealized losses		(2,009)	(2,552)
Estimated fair value		589,797	657,936
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	195,087	181,670
Gross unrealized gains		25	5
Gross unrealized losses		(161)	(250)
Estimated fair value		194,951	181,425
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	202,408	258,703
Gross unrealized gains		36	3
Gross unrealized losses		(1,174)	(1,705)
Estimated fair value		201,270	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]	26,548	50,559
Gross unrealized gains		0	1
Gross unrealized losses		(10)	(19)
Estimated fair value		26,538	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]	28,807	38,956
Gross unrealized gains		1	0
Gross unrealized losses		(167)	(244)
Estimated fair value		28,641	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]	29,298	2,604
Gross unrealized gains		2	0
Gross unrealized losses		(14)	(3)
Estimated fair value		29,286	2,601
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]	72,775	79,414
Gross unrealized gains		2	18
Gross unrealized losses		(134)	(88)
Estimated fair value		72,643	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]	36,816	48,552
Gross unrealized gains		1	3
Gross unrealized losses		(349)	(243)
Estimated fair value		36,468	48,312
Equity Securities [Member]
Summary of Investments [Abstract]
Cost		2,133	7,162
Gross unrealized gains		281	17,630
Gross unrealized losses		0	0
Estimated fair value		2,414	24,792
Equity Securities [Member] | Regulus Therapeutics Inc. [Member]
Summary of Investments [Abstract]
Cost		2,133	7,162
Gross unrealized gains		281	17,630
Gross unrealized losses		0	0
Estimated fair value		$ 2,414	$ 24,792

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

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