Marketable Securities (Amortized Cost And Fair Value Of Marketable Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 236,995	$ 213,804
Gross Unrealized Gains	70	25
Gross Unrealized Losses	(116)	(238)
Fair Value	236,949	213,591
Marketable securities have been in a continuous unrealized loss position for more than twelve months	0
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	130,833	177,810
Gross Unrealized Gains	40	24
Gross Unrealized Losses	(102)	(221)
Fair Value	130,771	177,613
Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,400	3,500
Fair Value	40,400	3,500
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,290
Gross Unrealized Gains	25
Fair Value	27,315
Markerable Securities - Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,228	3,491
Fair Value	20,228	3,491
U S Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,244	12,881
Gross Unrealized Gains	1
Gross Unrealized Losses	(14)	(10)
Fair Value	12,231	12,871
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,600	12,068
Gross Unrealized Gains	4
Gross Unrealized Losses		(3)
Fair Value	4,604	12,065
International Government Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,462
Gross Unrealized Losses		(3)
Fair Value		1,459
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,400	1,400
Gross Unrealized Losses		(1)
Fair Value	$ 1,400	1,399
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,192
Gross Unrealized Gains		1
Fair Value		$ 1,193

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

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-Publisher FASB
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