Investments, Summary of Investments (Details) - USD ($) $ in Thousands		Sep. 30, 2018		Dec. 31, 2017
Available-for-sale Securities [Member]
Summary of Investments [Abstract]
Cost	[1]	$ 1,502,739		$ 911,594
Gross unrealized gains		41		14
Gross unrealized losses		(3,942)		(3,072)
Estimated fair value		1,498,838		908,536
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	1,188,558		643,193
Gross unrealized gains		25		6
Gross unrealized losses		(1,852)		(1,103)
Estimated fair value		1,186,731		642,096
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	314,181		268,401
Gross unrealized gains		16		8
Gross unrealized losses		(2,090)		(1,969)
Estimated fair value		312,107		266,440
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	[1]	841,397	[2]	500,599
Gross unrealized gains		23		2
Gross unrealized losses		(1,264)		(752)
Estimated fair value		840,156		499,849
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member]
Summary of Investments [Abstract]
Cost	[1]	240,208		148,663
Gross unrealized gains		16		8
Gross unrealized losses		(1,420)		(1,059)
Estimated fair value		238,804		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]	143,008		83,926
Gross unrealized gains		2		0
Gross unrealized losses		(169)		(212)
Estimated fair value		142,841		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]	32,435		52,779
Gross unrealized gains		0		0
Gross unrealized losses		(109)		(168)
Estimated fair value		32,326		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]	154,280	[2]	29,428
Gross unrealized gains		0		0
Gross unrealized losses		(89)		(17)
Estimated fair value		154,191		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		9,872	[1]	1,409
Gross unrealized gains		0		0
Gross unrealized losses		(10)		(2)
Estimated fair value		9,862		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]	49,873		29,240
Gross unrealized gains		0		4
Gross unrealized losses		(330)		(122)
Estimated fair value		49,543		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]	31,666		65,550
Gross unrealized gains		0		0
Gross unrealized losses		(551)		(740)
Estimated fair value		$ 31,115		$ 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.