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Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 34,996	$ 16,528
Investments, amortized cost	530,794	353,774
Investments	528,465	351,975
Total amortized cost	515,162
Gross unrealized gains	1
Gross unrealized losses	(2,330)	(1,799)
Securities	512,833
Marketable securities
Schedule of Available-for-sale Securities
Securities	512,833	346,800
Money Market Funds
Schedule of Available-for-sale Securities
Cash and cash equivalents	15,632	5,175
Corporate Securities
Schedule of Available-for-sale Securities
Total amortized cost	98,498	123,860
Gross unrealized gains	1
Gross unrealized losses	(471)	(590)
Securities	98,028	123,270
Government Securities
Schedule of Available-for-sale Securities
Total amortized cost	416,664	224,739
Gross unrealized losses	(1,859)	(1,209)
Securities	414,805	223,530
Cash and Cash Equivalents
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 15,632	$ 5,175

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