Marketable Securities Schedule of Investments in Available For Sale Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	$ 2,743,029	$ 2,027,272
Unrealized Gains	2,312	2,210
Unrealized Losses	(20,584)	(8,926)
Fair Value	2,724,757	2,020,556
Corporate bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	2,497,276	1,717,976
Unrealized Gains	2,282	2,176
Unrealized Losses	(18,987)	(7,672)
Fair Value	2,480,571	1,712,480
U.S. government and government agency obligations \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	126,395	186,699
Unrealized Gains	30	34
Unrealized Losses	(1,589)	(1,241)
Fair Value	124,836	185,492
Municipal bonds \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	2,572	4,600
Unrealized Gains	0	0
Unrealized Losses	(8)	(13)
Fair Value	2,564	4,587
Commercial paper \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	76,013	106,973
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	76,013	106,973
Certificates of deposit \| Unrestricted
Debt Securities, Available-for-sale [Line Items]
Amortized Cost Basis	40,773	11,024
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	$ 40,773	$ 11,024

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of 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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