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Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 42,707 $ 38,667
Marketable Securities Fair Value, Total 186,449 144,628
Debt Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 46,686 34,498
Available-for-sale Securities, Gross Unrealized Gain 1,739 1,256
Federal Agency Mortgage Backed Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 6,662 7,430
Available-for-sale Securities, Gross Unrealized Gain 272 319
U.S. Government Bonds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 13,846 12,237
Available-for-sale Securities, Gross Unrealized Gain 311 138
Municipal Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 5,197 2,481
Available-for-sale Securities, Gross Unrealized Gain 161 144
Corporate Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 20,981 12,350
Available-for-sale Securities, Gross Unrealized Gain 995 655
Common Stock    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 53,489 61,884
Available-for-sale Securities, Gross Unrealized Gain 30,766 34,066
Equity Mutual Funds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 54,236 42,244
Available-for-sale Securities, Gross Unrealized Gain 7,028 3,345
Institutional Funds-International Equity    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 26,149 0
Available-for-sale Securities, Gross Unrealized Gain 3,174 0
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 $ 5,889 $ 6,002