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Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 873 $ 53,130
Marketable Securities Fair Value, Total 24,191 194,867
Debt Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 24,191 39,134
Available-for-sale Securities, Gross Unrealized Gain 873 1,635
Federal Agency Mortgage Backed Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 9,469 5,933
Available-for-sale Securities, Gross Unrealized Gain 143 203
U.S. Government Bonds    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 2,205 11,129
Available-for-sale Securities, Gross Unrealized Gain 92 256
Municipal Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 1,073 2,558
Available-for-sale Securities, Gross Unrealized Gain 92 109
Corporate Obligations    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 11,444 19,514
Available-for-sale Securities, Gross Unrealized Gain 546 1,067
Equity Securities | Domestic Equity Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 0 120,065
Available-for-sale Securities, Gross Unrealized Gain 0 45,587
Equity Securities | International Equity Securities    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Unrealized Gain Position, Fair Value 0 28,804
Available-for-sale Securities, Gross Unrealized Gain 0 5,908
Cash | Held in trust Decommissioning Fund Investment    
Financial Instruments and Investments [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain 0 0
Cash $ 0 $ 6,864