Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Mar. 31, 2018		Dec. 31, 2017
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	$ 848,653		$ 911,594
Gross unrealized gains		8		14
Gross unrealized losses		(4,600)		(3,072)
Estimated fair value		844,061		908,536
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	648,232		643,193
Gross unrealized gains		3		6
Gross unrealized losses		(1,846)		(1,103)
Estimated fair value		646,389		642,096
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	200,421		268,401
Gross unrealized gains		5		8
Gross unrealized losses		(2,754)		(1,969)
Estimated fair value		197,672		266,440
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	433,297	[2]	500,599
Gross unrealized gains		1		2
Gross unrealized losses		(1,286)		(752)
Estimated fair value		432,012		499,849
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	126,128		148,663
Gross unrealized gains		5		8
Gross unrealized losses		(1,807)		(1,059)
Estimated fair value		124,326		147,612
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	129,235		83,926
Gross unrealized gains		0		0
Gross unrealized losses		(323)		(212)
Estimated fair value		128,912		83,714
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	21,547		52,779
Gross unrealized gains		0		0
Gross unrealized losses		(162)		(168)
Estimated fair value		21,385		52,611
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	54,076	[2]	29,428
Gross unrealized gains		1		0
Gross unrealized losses		(43)		(17)
Estimated fair value		54,034		29,411
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost				1,409
Gross unrealized gains				0
Gross unrealized losses				(2)
Estimated fair value				1,407
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]	31,624		29,240
Gross unrealized gains		1		4
Gross unrealized losses		(194)		(122)
Estimated fair value		31,431		29,122
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]	52,746		65,550
Gross unrealized gains		0		0
Gross unrealized losses		(785)		(740)
Estimated fair value		$ 51,961		$ 64,810

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

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