Marketable Securities - Continuous Loss Position (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Long-term AFS securities
Other than temporary impairment, loss	$ 0
US Treasury Securities [Member]
Short-term AFS securities
Fair value, less than 12 months	70,951,000
Gross unrealized losses, less than 12 months	(34,000)
Fair value, 12 months or greater	0
Gross unrealized losses, 12 months or greater	0
Fair value, total	70,951,000
Gross unrealized losses, total	(34,000)
Long-term AFS securities
Fair value, less than 12 months	52,628,000
Gross unrealized losses, less than 12 months	(403,000)
Fair value, 12 months or greater	0
Gross unrealized losses, 12 months or greater	0
Fair value, total	52,628,000
Gross unrealized losses, total	(403,000)
Corporate Debt Securities [Member]
Short-term AFS securities
Fair value, less than 12 months	140,111,000
Gross unrealized losses, less than 12 months	(199,000)
Fair value, 12 months or greater	9,996,000
Gross unrealized losses, 12 months or greater	(28,000)
Fair value, total	150,107,000
Gross unrealized losses, total	(227,000)
Long-term AFS securities
Fair value, less than 12 months	81,118,000
Gross unrealized losses, less than 12 months	(579,000)
Fair value, 12 months or greater	10,886,000
Gross unrealized losses, 12 months or greater	(104,000)
Fair value, total	92,004,000
Gross unrealized losses, total	(683,000)
US Government Agencies Securities [Member]
Short-term AFS securities
Fair value, less than 12 months	19,770,000
Gross unrealized losses, less than 12 months	(50,000)
Fair value, 12 months or greater	25,870,000
Gross unrealized losses, 12 months or greater	(129,000)
Fair value, total	45,640,000
Gross unrealized losses, total	(179,000)
Long-term AFS securities
Fair value, less than 12 months	23,998,000
Gross unrealized losses, less than 12 months	(125,000)
Fair value, 12 months or greater	72,781,000
Gross unrealized losses, 12 months or greater	(894,000)
Fair value, total	96,779,000
Gross unrealized losses, total	(1,019,000)
Short-term Available-for Sale Securities [Member]
Short-term AFS securities
Fair value, less than 12 months	230,832,000
Gross unrealized losses, less than 12 months	(283,000)
Fair value, 12 months or greater	35,866,000
Gross unrealized losses, 12 months or greater	(157,000)
Fair value, total	266,698,000
Gross unrealized losses, total	(440,000)
Long-term AFS securities [Member]
Long-term AFS securities
Fair value, less than 12 months	157,744,000
Gross unrealized losses, less than 12 months	(1,107,000)
Fair value, 12 months or greater	83,667,000
Gross unrealized losses, 12 months or greater	(998,000)
Fair value, total	241,411,000
Gross unrealized losses, total	$ (2,105,000)