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Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 38,667 $ 38,366
Marketable Securities Fair Value, Total 144,628 140,096
Debt Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 34,498 40,403
Available-for-sale Securities, Gross Unrealized Gain 1,256 1,274
Collateralized Mortgage Backed Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 7,430 9,589
Available-for-sale Securities, Gross Unrealized Gain 319 438
U.S. Government Bonds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 12,237 12,033
Available-for-sale Securities, Gross Unrealized Gain 138 136
Municipal Bonds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 2,481 8,671
Available-for-sale Securities, Gross Unrealized Gain 144 332
Corporate Debt Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 12,350 10,110
Available-for-sale Securities, Gross Unrealized Gain 655 368
Equity Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 61,884 72,636
Available-for-sale Securities, Gross Unrealized Gain 34,066 37,001
Equity Mutual Funds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 42,244 18,853
Available-for-sale Securities, Gross Unrealized Gain 3,345 91
Cash    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain 0 0
Cash | Held in trust Decommissioning Fund Investment    
Financial Instruments and Investments [Line Items]    
Cash $ 6,002 $ 8,204