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Financial Instruments And Investments (Marketable Securities Fair Value and Unrecognized Gross Unrealized Gains) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Gross Unrealized Gain $ 667      
Marketable Securities Fair Value, Total 27,181 $ 194,867    
Cash and Cash Equivalents 12,900 6,990 $ 8,420 $ 8,149
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   53,130    
Debt Securities        
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Unrealized Gain Position, Fair Value 27,181 39,134    
Available-for-sale Securities, Gross Unrealized Gain 667 1,635    
Federal Agency Mortgage Backed Securities        
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Unrealized Gain Position, Fair Value 9,959 5,933    
Available-for-sale Securities, Gross Unrealized Gain 176 203    
U.S. Government Bonds        
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Unrealized Gain Position, Fair Value 6,987 11,129    
Available-for-sale Securities, Gross Unrealized Gain 149 256    
Municipal Bonds        
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Unrealized Gain Position, Fair Value 1,952 2,558    
Available-for-sale Securities, Gross Unrealized Gain 120 109    
Corporate Debt Obligations        
Financial Instruments and Investments [Line Items]        
Available-for-sale Securities, Unrealized Gain Position, Fair Value 8,283 19,514    
Available-for-sale Securities, Gross Unrealized Gain 222 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 Equity Securities, Accumulated Gross Unrealized Gain, before Tax 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 Equity Securities, Accumulated Gross Unrealized Gain, before Tax 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 and Cash Equivalents $ 0 $ 6,864