Financial Instruments And Investments (Marketable Securities Fair Value And Unrealized Losses) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) securities		Dec. 31, 2016 USD ($) securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		$ 49,503	[1]	$ 86,509	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(852)	[1]	(1,844)	[2]
Marketable Securities Fair Value 12 Months or Longer		42,496	[1]	24,571	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(2,207)	[1]	(1,401)	[2]
Marketable Securities Fair Value, Total		91,999	[1]	111,080	[2]
Marketable Securities Unrealized Losses, Total		$ (3,059)	[1]	$ (3,245)	[2]
Number of securities | securities		146		152
Debt Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		$ 48,541	[1]	$ 60,804	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(642)	[1]	(1,567)	[2]
Marketable Securities Fair Value 12 Months or Longer		42,496	[1]	24,571	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(2,207)	[1]	(1,401)	[2]
Marketable Securities Fair Value, Total		91,037	[1]	85,375	[2]
Marketable Securities Unrealized Losses, Total		(2,849)	[1]	(2,968)	[2]
Collateralized Mortgage Backed Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		4,700	[1]	11,582	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(46)	[1]	(239)	[2]
Marketable Securities Fair Value 12 Months or Longer		10,099	[1]	436	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(165)	[1]	(22)	[2]
Marketable Securities Fair Value, Total		14,799	[1]	12,018	[2]
Marketable Securities Unrealized Losses, Total		(211)	[1]	(261)	[2]
U.S. Government Bonds
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		28,866	[1]	31,655	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(416)	[1]	(762)	[2]
Marketable Securities Fair Value 12 Months or Longer		18,186	[1]	17,976	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(969)	[1]	(835)	[2]
Marketable Securities Fair Value, Total		47,052	[1]	49,631	[2]
Marketable Securities Unrealized Losses, Total		(1,385)	[1]	(1,597)	[2]
Municipal Bonds
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		4,290	[1]	9,596	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(73)	[1]	(394)	[2]
Marketable Securities Fair Value 12 Months or Longer		9,736	[1]	4,067	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(742)	[1]	(372)	[2]
Marketable Securities Fair Value, Total		14,026	[1]	13,663	[2]
Marketable Securities Unrealized Losses, Total		(815)	[1]	(766)	[2]
Corporate Debt Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		10,685	[1]	7,971	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(107)	[1]	(172)	[2]
Marketable Securities Fair Value 12 Months or Longer		4,475	[1]	2,092	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		(331)	[1]	(172)	[2]
Marketable Securities Fair Value, Total		15,160	[1]	10,063	[2]
Marketable Securities Unrealized Losses, Total		(438)	[1]	(344)	[2]
Equity Securities
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months		962	[1]	2,760	[2]
Marketable Securities Unrealized Losses Less Than 12 Months		(210)	[1]	(167)	[2]
Marketable Securities Fair Value 12 Months or Longer		0	[1]	0	[2]
Marketable Securities Unrealized Losses 12 Months or Longer		0	[1]	0	[2]
Marketable Securities Fair Value, Total		962	[1]	2,760	[2]
Marketable Securities Unrealized Losses, Total		$ (210)	[1]	(167)	[2]
Institutional Funds-International Equity
Financial Instruments and Investments [Line Items]
Marketable Securities Fair Value, Less than 12 Months	[2]			22,945
Marketable Securities Unrealized Losses Less Than 12 Months	[2]			(110)
Marketable Securities Fair Value 12 Months or Longer	[2]			0
Marketable Securities Unrealized Losses 12 Months or Longer	[2]			0
Marketable Securities Fair Value, Total	[2]			22,945
Marketable Securities Unrealized Losses, Total	[2]			$ (110)

[1] Includes approximately 146 securities.

[2] Includes approximately 152 securities.