Marketable Securities (Summary of Securities Held) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 760,306	$ 690,278
Gross Unrealized Holding Gains	1,812	24
Gross Unrealized Holding Losses	(478)	(997)
Estimated Fair Value	761,640	689,305
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	760,306
Gross Unrealized Holding Gains	37
Gross Unrealized Holding Losses	(478)
Estimated Fair Value	759,865
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	238,092	254,396
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Estimated Fair Value	238,092	254,396
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	188,009	133,062
Gross Unrealized Holding Gains	32	23
Gross Unrealized Holding Losses	(237)	(336)
Estimated Fair Value	187,804	132,749
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	155,421	87,173
Gross Unrealized Holding Gains	1	1
Gross Unrealized Holding Losses	(116)	(63)
Estimated Fair Value	155,306	87,111
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,503	25,244
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	(20)	(94)
Estimated Fair Value	30,483	25,150
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	133,530	163,403
Gross Unrealized Holding Gains	4	0
Gross Unrealized Holding Losses	(105)	(504)
Estimated Fair Value	133,429	162,899
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,751	27,000
Gross Unrealized Holding Gains	0	0
Gross Unrealized Holding Losses	0	0
Estimated Fair Value	14,751	$ 27,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0
Gross Unrealized Holding Gains	1,775
Gross Unrealized Holding Losses	0
Estimated Fair Value	$ 1,775

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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