Marketable Securities (Amortized Cost And Fair Value Of Marketable Securities) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 284,979	$ 236,995
Gross Unrealized Gains	163	70
Gross Unrealized Losses	(57)	(116)
Fair Value	285,085	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	139,533	130,833
Gross Unrealized Gains	129	40
Gross Unrealized Losses	(31)	(102)
Fair Value	139,631	130,771
Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,473	40,400
Fair Value	60,473	40,400
Commercial Paper - Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,234	20,228
Fair Value	24,234	20,228
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,476	27,290
Gross Unrealized Gains	11	25
Gross Unrealized Losses	(6)
Fair Value	22,481	27,315
U.S. Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,282	12,244
Gross Unrealized Gains	11	1
Gross Unrealized Losses	(7)	(14)
Fair Value	16,286	12,231
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,992
Gross Unrealized Gains	1
Fair Value	10,993
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,989	4,600
Gross Unrealized Gains	11	4
Gross Unrealized Losses	(13)
Fair Value	$ 10,987	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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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