Marketable Securities Available-for-sale Securities - Continuous Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Total short-term AFS securities
Short-term AFS securities
Fair value, less than 12 months	$ 178,371	$ 230,832
Gross unrealized losses, less than 12 months	(182)	(283)
Fair value, greater than 12 months	136,020	35,866
Gross unrealized losses, greater than 12 months	(1,117)	(157)
Fair value, total	314,391	266,698
Gross unrealized losses, total	(1,299)	(440)
Total long-term AFS securities
Long-term AFS securities
Fair value, less than 12 months	167,388	157,744
Gross unrealized losses, less than 12 months	(1,082)	(1,107)
Fair value, greater than 12 months	57,335	83,667
Gross unrealized losses, greater than 12 months	(963)	(998)
Fair value, total	224,723	241,411
Gross unrealized losses, total	(2,045)	(2,105)
Corporate Debt Securities
Short-term AFS securities
Fair value, less than 12 months	82,866	140,111
Gross unrealized losses, less than 12 months	(123)	(199)
Fair value, greater than 12 months	41,587	9,996
Gross unrealized losses, greater than 12 months	(227)	(28)
Fair value, total	124,453	150,107
Gross unrealized losses, total	(350)	(227)
Long-term AFS securities
Fair value, less than 12 months	94,522	81,118
Gross unrealized losses, less than 12 months	(459)	(579)
Fair value, greater than 12 months	28,379	10,886
Gross unrealized losses, greater than 12 months	(500)	(104)
Fair value, total	122,901	92,004
Gross unrealized losses, total	(959)	(683)
US Treasury Securities
Short-term AFS securities
Fair value, less than 12 months	92,021	70,951
Gross unrealized losses, less than 12 months	(56)	(34)
Fair value, greater than 12 months	20,763	0
Gross unrealized losses, greater than 12 months	(176)	0
Fair value, total	112,784	70,951
Gross unrealized losses, total	(232)	(34)
Long-term AFS securities
Fair value, less than 12 months	35,178	23,998
Gross unrealized losses, less than 12 months	(305)	(125)
Fair value, greater than 12 months	18,754	72,781
Gross unrealized losses, greater than 12 months	(323)	(894)
Fair value, total	53,932	96,779
Gross unrealized losses, total	(628)	(1,019)
U.S. governmental agency securities
Short-term AFS securities
Fair value, less than 12 months	3,484	19,770
Gross unrealized losses, less than 12 months	(3)	(50)
Fair value, greater than 12 months	73,670	25,870
Gross unrealized losses, greater than 12 months	(714)	(129)
Fair value, total	77,154	45,640
Gross unrealized losses, total	(717)	(179)
Long-term AFS securities
Fair value, less than 12 months	37,688	52,628
Gross unrealized losses, less than 12 months	(318)	(403)
Fair value, greater than 12 months	10,202	0
Gross unrealized losses, greater than 12 months	(140)	0
Fair value, total	47,890	52,628
Gross unrealized losses, total	$ (458)	$ (403)