Marketable Securities (Amortized Cost And Fair Value Of Marketable Securities) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 239,963	$ 236,995
Gross Unrealized Gains	83	70
Gross Unrealized Losses	(113)	(116)
Fair Value	239,933	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	120,898	130,833
Gross Unrealized Gains	76	40
Gross Unrealized Losses	(81)	(102)
Fair Value	120,893	130,771
Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,198	40,400
Fair Value	43,198	40,400
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,610	27,290
Gross Unrealized Gains	6	25
Gross Unrealized Losses	(16)
Fair Value	37,600	27,315
Commercial Paper - Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,013	20,228
Fair Value	17,013	20,228
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,644	12,244
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(15)	(14)
Fair Value	16,630	12,231
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,600	4,600
Gross Unrealized Gains		4
Gross Unrealized Losses	(1)
Fair Value	$ 4,599	4,604
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,400
Fair Value		$ 1,400

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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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