Marketable Securities (Amortized Cost And Fair Value Of Marketable Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 284,429	$ 236,995
Gross Unrealized Gains	120	70
Gross Unrealized Losses	(294)	(116)
Fair Value	284,255	236,949
Marketable securities that we consider to be other-than-temporarily impaired	0
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	140,969	130,833
Gross Unrealized Gains	96	40
Gross Unrealized Losses	(179)	(102)
Fair Value	140,886	130,771
Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,510	40,400
Fair Value	58,510	40,400
Markerable Securities - Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,171	20,228
Fair Value	29,171	20,228
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,206	27,290
Gross Unrealized Gains	4	25
Gross Unrealized Losses	(43)
Fair Value	22,167	27,315
U S Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,619	12,244
Gross Unrealized Gains	11	1
Gross Unrealized Losses	(19)	(14)
Fair Value	15,611	12,231
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,990	4,600
Gross Unrealized Gains	9	4
Gross Unrealized Losses	(52)
Fair Value	10,947	4,604
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,400
Fair Value		$ 1,400
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
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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