Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Jun. 30, 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]	$ 735,735		591,739
Gross unrealized gains		81		67
Gross unrealized losses		(1,725)		(2,009)
Estimated fair value		734,091		589,797
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	507,364		323,708
Gross unrealized gains		39		29
Gross unrealized losses		(607)		(319)
Estimated fair value		506,796		323,418
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	228,371		268,031
Gross unrealized gains		42		38
Gross unrealized losses		(1,118)		(1,690)
Estimated fair value		227,295		266,379
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	369,416	[2]	195,087
Gross unrealized gains		27		25
Gross unrealized losses		(360)		(161)
Estimated fair value		369,083		194,951
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	159,233		202,408
Gross unrealized gains		42		36
Gross unrealized losses		(674)		(1,174)
Estimated fair value		158,601		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]	63,125		26,548
Gross unrealized gains		0		0
Gross unrealized losses		(98)		(10)
Estimated fair value		63,027		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]	19,181		28,807
Gross unrealized gains		0		1
Gross unrealized losses		(108)		(167)
Estimated fair value		19,073		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]	28,295	[2]	29,298
Gross unrealized gains		0		2
Gross unrealized losses		(25)		(14)
Estimated fair value		28,270		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]	0
Gross unrealized gains		0
Gross unrealized losses		0
Estimated fair value		0
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]	43,528		72,775
Gross unrealized gains		12		2
Gross unrealized losses		(122)		(134)
Estimated fair value		43,418		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]	49,957		36,816
Gross unrealized gains		0		1
Gross unrealized losses		(336)		(349)
Estimated fair value		49,621		36,468
Other Municipal Debt Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	3,000
Gross unrealized gains		0
Gross unrealized losses		(2)
Estimated fair value		$ 2,998
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.