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Investments In Securities (Schedule of Equity Securities) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cost $ 26,530 $ 26,003
Estimated Fair Value 47,252 47,367
Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cost 26,530 26,003
Gross Unrealized Gains 20,922 21,376
Gross Unrealized Losses 200 12
Estimated Fair Value $ 47,252 $ 47,367