Financial Instruments And Investments (Marketable Securities Fair Value And Unrealized Losses) (Details) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018 USD ($) securities		Dec. 31, 2017 USD ($) securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months	[1]			$ 49,503
Marketable Securities Unrealized Losses Less Than 12 Months	[1]			(852)
Marketable Securities Fair Value 12 Months or Longer	[1]			42,496
Marketable Securities Unrealized Losses 12 Months or Longer	[1]			(2,207)
Marketable Securities Fair Value Total	[1]			91,999
Marketable Securities Unrealized Losses Total	[1]			$ (3,059)
Number of securities | securities		147		146
Debt Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		$ 65,095	[2]	$ 48,541	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(1,772)	[2]	(642)	[1]
Marketable Securities Fair Value 12 Months or Longer		39,501	[2]	42,496	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(2,504)	[2]	(2,207)	[1]
Marketable Securities Fair Value Total		104,596	[2]	91,037	[1]
Marketable Securities Unrealized Losses Total		(4,276)	[2]	(2,849)	[1]
Federal Agency Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		5,995	[2]	4,700	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(114)	[2]	(46)	[1]
Marketable Securities Fair Value 12 Months or Longer		10,376	[2]	10,099	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(407)	[2]	(165)	[1]
Marketable Securities Fair Value Total		16,371	[2]	14,799	[1]
Marketable Securities Unrealized Losses Total		(521)	[2]	(211)	[1]
U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		34,018	[2]	28,866	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(940)	[2]	(416)	[1]
Marketable Securities Fair Value 12 Months or Longer		18,325	[2]	18,186	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(1,215)	[2]	(969)	[1]
Marketable Securities Fair Value Total		52,343	[2]	47,052	[1]
Marketable Securities Unrealized Losses Total		(2,155)	[2]	(1,385)	[1]
Municipal Obligations
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		4,227	[2]	4,290	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(109)	[2]	(73)	[1]
Marketable Securities Fair Value 12 Months or Longer		7,031	[2]	9,736	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(581)	[2]	(742)	[1]
Marketable Securities Fair Value Total		11,258	[2]	14,026	[1]
Marketable Securities Unrealized Losses Total		(690)	[2]	(815)	[1]
Corporate Obligations
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months		20,855	[2]	10,685	[1]
Marketable Securities Unrealized Losses Less Than 12 Months		(609)	[2]	(107)	[1]
Marketable Securities Fair Value 12 Months or Longer		3,769	[2]	4,475	[1]
Marketable Securities Unrealized Losses 12 Months or Longer		(301)	[2]	(331)	[1]
Marketable Securities Fair Value Total		24,624	[2]	15,160	[1]
Marketable Securities Unrealized Losses Total		$ (910)	[2]	(438)	[1]
Domestic Equity Securities | Equity Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value Less than 12 Months	[1]			962
Marketable Securities Unrealized Losses Less Than 12 Months	[1]			(210)
Marketable Securities Fair Value 12 Months or Longer	[1]			0
Marketable Securities Unrealized Losses 12 Months or Longer	[1]			0
Marketable Securities Fair Value Total	[1]			962
Marketable Securities Unrealized Losses Total	[1]			$ (210)

[1] Includes 146 securities.

[2] Includes 147 securities.