Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 35,292	$ 16,528
Investments, amortized cost	553,229	353,774
Investments	551,231	351,975
Total amortized cost	522,085
Gross unrealized gains	21
Gross unrealized losses	(2,019)	(1,799)
Securities	520,087
Marketable securities
Schedule of Available-for-sale Securities
Securities	520,087	346,800
Money Market Funds
Schedule of Available-for-sale Securities
Cash and cash equivalents	31,144	5,175
Corporate Securities
Schedule of Available-for-sale Securities
Total amortized cost	96,756	123,860
Gross unrealized losses	(654)	(590)
Securities	96,102	123,270
Government Securities
Schedule of Available-for-sale Securities
Total amortized cost	425,329	224,739
Gross unrealized gains	21
Gross unrealized losses	(1,365)	(1,209)
Securities	423,985	223,530
Cash and Cash Equivalents
Schedule of Available-for-sale Securities
Cash and cash equivalents	$ 31,144	$ 5,175

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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