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Earnings Per Common Share And Share Awards Summary Of Share-Based Award Transactions (Details) - SARs - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Number Of Shares, Outstanding Beginning Balance 25,000 25,000 25,000  
Number Of Shares, SARs granted 0 0 4,000  
Number Of Shares, SARS exercised (1,000)   (4,000)  
Number Of Shares, Outstanding Ending Balance 24,000   25,000 25,000
Number Of Shares, Exercisable as of March 31, 2018 24,000      
Number Of Shares, Unvested as of March 31, 2018 0      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]        
Weighted Average Exercise Price, Outstanding Beginning Balance $ 65.85 $ 3.85 $ 3.85  
Weighted Average Exercise Price, SARs granted 0.00   93.87  
Weighted Average Exercise Price, SARS exercised 41.50   32.00  
Weighted Average Exercise Price, Outstanding Ending Balance 95.56   $ 65.85 $ 3.85
Weighted Average Exercise Price, Exercisable as of March 31, 2018 $ 95.56      
Average Remaining Contractual Term, Outstanding Beginning Balance 3 years 10 months 17 days   3 years 10 months 6 days 3 years 10 months 6 days
Average Remaining Contractual Term, Outstanding Ending Balance 3 years 10 months 17 days   3 years 10 months 6 days 3 years 10 months 6 days
Average Remaining Contractual Term, Exercisable as of March 31, 2018 3 years 10 months 17 days      
Aggregate Intrinsic Value, Outstanding Beginning Balance $ 837 $ 837 $ 837  
Aggregate Intrinsic Value, Outstanding Ending Balance 2,504   $ 837 $ 837
Aggregate Intrinsic Value, Exercisable as of March 31, 2018 $ 2,504      
Share Based Compensation Arrangement By Share Based Payment Award, Unvested, Weighted Average Exercise Price $ 0.00      
Average Remaining Contractual Term, Unvested as of March 31, 2018 0 days      
Aggregate Intrinsic Value, Unvested as of March 31, 2018 $ 0