Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Dec. 31, 2018		Dec. 31, 2017
Summary of Investments [Abstract]
Cost		$ 1,874,529
Gross unrealized gains		556
Gross unrealized losses		(4,068)
Estimated fair value		1,871,017
Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	1,433,930		$ 643,193
Gross unrealized gains		31		6
Gross unrealized losses		(2,362)		(1,103)
Estimated fair value		1,431,599		642,096
Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	439,387		268,401
Gross unrealized gains		388		8
Gross unrealized losses		(1,706)		(1,969)
Estimated fair value		438,069		266,440
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	1,873,317		911,594
Gross unrealized gains		419		14
Gross unrealized losses		(4,068)		(3,072)
Estimated fair value		1,869,668		908,536
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1],[2]	956,879		500,599
Gross unrealized gains		13		2
Gross unrealized losses		(1,858)		(752)
Estimated fair value		955,034		499,849
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	299,018		148,663
Gross unrealized gains		194		8
Gross unrealized losses		(1,286)		(1,059)
Estimated fair value		297,926		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]	168,839		83,926
Gross unrealized gains		3		0
Gross unrealized losses		(104)		(212)
Estimated fair value		168,738		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]	107,789		52,779
Gross unrealized gains		194		0
Gross unrealized losses		(109)		(168)
Estimated fair value		107,874		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]	244,640	[2]	29,428
Gross unrealized gains		15		0
Gross unrealized losses		(77)		(17)
Estimated fair value		244,578		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]	15,600		1,409
Gross unrealized gains		0		0
Gross unrealized losses		(24)		(2)
Estimated fair value		15,576		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]	63,572		29,240	[2]
Gross unrealized gains		0		4
Gross unrealized losses		(323)		(122)
Estimated fair value		63,249		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]	16,980		65,550
Gross unrealized gains		0		0
Gross unrealized losses		(287)		(740)
Estimated fair value		16,693		$ 64,810
Equity Securities [Member]
Summary of Investments [Abstract]
Cost	[3]	1,212
Gross unrealized gains		137
Gross unrealized losses		0
Estimated fair value		$ 1,349

[1] We hold our available-for-sale securities at amortized cost.

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

[3] We recognize our equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer on our consolidated balance sheet.