Marketable Securities (Amortized Cost And Fair Value Of Marketable Securities) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 284,429
Gross Unrealized Gains	120
Gross Unrealized Losses	(294)
Fair Value	284,255
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	140,969
Gross Unrealized Gains	96
Gross Unrealized Losses	(179)
Fair Value	140,886
Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,510
Fair Value	58,510
Commercial Paper - Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,171
Fair Value	29,171
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,206
Gross Unrealized Gains	4
Gross Unrealized Losses	(43)
Fair Value	22,167
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,619
Gross Unrealized Gains	11
Gross Unrealized Losses	(19)
Fair Value	15,611
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,990
Gross Unrealized Gains	9
Gross Unrealized Losses	(52)
Fair Value	10,947
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,964
Gross Unrealized Losses	(1)
Fair Value	$ 6,963

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
-SubTopic 10
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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