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Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 43,608 $ 38,667
Marketable Securities Fair Value, Total 185,532 144,628
Debt Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 41,281 34,498
Available-for-sale Securities, Gross Unrealized Gain 1,328 1,256
Federal Agency Mortgage Backed Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 6,991 7,430
Available-for-sale Securities, Gross Unrealized Gain 276 319
U.S. Government Bonds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 15,811 12,237
Available-for-sale Securities, Gross Unrealized Gain 198 138
Municipal Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 4,086 2,481
Available-for-sale Securities, Gross Unrealized Gain 144 144
Corporate Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 14,393 12,350
Available-for-sale Securities, Gross Unrealized Gain 710 655
Common Stock    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 62,487 61,884
Available-for-sale Securities, Gross Unrealized Gain 34,868 34,066
Mutual Funds - Equity    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 50,225 42,244
Available-for-sale Securities, Gross Unrealized Gain 5,690 3,345
Institutional Funds-International Equity    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 24,737 0
Available-for-sale Securities, Gross Unrealized Gain 1,722 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 $ 6,802 $ 6,002