Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Mar. 31, 2017		Dec. 31, 2016
Summary of Investments [Abstract]
Cost				$ 593,872
Gross unrealized gains				348
Gross unrealized losses				(2,009)
Estimated fair value				592,211
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	$ 758,372		591,739
Gross unrealized gains		87		67
Gross unrealized losses		(1,861)		(2,009)
Estimated fair value		756,598		589,797
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	484,952		323,708
Gross unrealized gains		22		29
Gross unrealized losses		(612)		(319)
Estimated fair value		484,362		323,418
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	273,420		268,031
Gross unrealized gains		65		38
Gross unrealized losses		(1,249)		(1,690)
Estimated fair value		272,236		266,379
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	341,043	[2]	195,087
Gross unrealized gains		18		25
Gross unrealized losses		(405)		(161)
Estimated fair value		340,656		194,951
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	189,152		202,408
Gross unrealized gains		49		36
Gross unrealized losses		(833)		(1,174)
Estimated fair value		188,368		201,270
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	64,784		26,548
Gross unrealized gains		2		0
Gross unrealized losses		(72)		(10)
Estimated fair value		64,714		26,538
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	25,670		28,807
Gross unrealized gains		0		1
Gross unrealized losses		(124)		(167)
Estimated fair value		25,546		28,641
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	23,297	[2]	29,298
Gross unrealized gains		0		2
Gross unrealized losses		(19)		(14)
Estimated fair value		23,278		29,286
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	3,497
Gross unrealized gains		0
Gross unrealized losses		(1)
Estimated fair value		3,496
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]	55,828		72,775
Gross unrealized gains		2		2
Gross unrealized losses		(116)		(134)
Estimated fair value		55,714		72,643
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]	55,101		36,816
Gross unrealized gains		16		1
Gross unrealized losses		(291)		(349)
Estimated fair value		$ 54,826		36,468
Equity Securities [Member]
Summary of Investments [Abstract]
Cost				2,133
Gross unrealized gains				281
Gross unrealized losses				0
Estimated fair value				2,414
Regulus Therapeutics Inc. [Member]
Summary of Investments [Abstract]
Cost				2,133
Gross unrealized gains				281
Gross unrealized losses				0
Estimated fair value				$ 2,414

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

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