Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Jun. 30, 2018		Dec. 31, 2017
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	$ 1,305,704		$ 911,594
Gross unrealized gains		42		14
Gross unrealized losses		(4,095)		(3,072)
Estimated fair value		1,301,651		908,536
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	1,053,279		643,193
Gross unrealized gains		14		6
Gross unrealized losses		(1,667)		(1,103)
Estimated fair value		1,051,626		642,096
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	252,425		268,401
Gross unrealized gains		28		8
Gross unrealized losses		(2,428)		(1,969)
Estimated fair value		250,025		266,440
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	747,172	[2]	500,599
Gross unrealized gains		2		2
Gross unrealized losses		(1,210)		(752)
Estimated fair value		745,964		499,849
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	199,673		148,663
Gross unrealized gains		28		8
Gross unrealized losses		(1,607)		(1,059)
Estimated fair value		198,094		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]	137,915		83,926
Gross unrealized gains		5		0
Gross unrealized losses		(233)		(212)
Estimated fair value		137,687		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]	3,986		52,779
Gross unrealized gains		0		0
Gross unrealized losses		(72)		(168)
Estimated fair value		3,914		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]	138,778	[2]	29,428
Gross unrealized gains		7		0
Gross unrealized losses		(34)		(17)
Estimated fair value		138,751		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]	29,414		29,240
Gross unrealized gains		0		4
Gross unrealized losses		(190)		(122)
Estimated fair value		29,224		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]	48,766		65,550
Gross unrealized gains		0		0
Gross unrealized losses		(749)		(740)
Estimated fair value		$ 48,017		$ 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.