Financial Instruments - Amortized to fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 334,479	$ 473,956
Available-for-sale debt securities, amortized cost	518,380	723,580
Gross Unrealized Gains	133	1
Gross Unrealized Losses	(11)	(132)
Available for sale securities	518,502	723,449
Marketable Securities	184,023	249,493
Debt Securities, Available-for-sale, Amortized Cost	518,380	723,580
U.S. government agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	30,375	9,738
Available-for-sale debt securities, amortized cost	77,316	72,884
Gross Unrealized Gains	48	1
Gross Unrealized Losses	0	(45)
Available for sale securities	77,364	72,840
Marketable Securities	46,989	63,102
Debt Securities, Available-for-sale, Amortized Cost	77,316	72,884
Corporate debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	70,103	42,562
Available-for-sale debt securities, amortized cost	207,063	229,040
Gross Unrealized Gains	85	0
Gross Unrealized Losses	(11)	(87)
Available for sale securities	207,137	228,953
Marketable Securities	137,034	186,391
Debt Securities, Available-for-sale, Amortized Cost	207,063	229,040
Cash [Member]
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	126,293	148,110
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Marketable Securities	0	0
Money market funds [Member]
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	107,708	273,546
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Marketable Securities	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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