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Pension and Other Postretirement Plans (Summary of Stock Option Activity) (Details) - $ / shares
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Outstanding, Beginning Balance - Number of Option Shares 889,300 575,000 575,000  
Options Granted - Number of Option Shares      
Options Exercised - Number of Option Shares (200) (700)    
Options Forfeited - Number of Option Shares   (35,000)    
Options Expired - Number of Option Shares      
Outstanding, Ending Balance - Number of Option Shares 889,100 539,300 889,300 575,000
Outstanding Option Shares, Range of Exercise Prices, Lower Range Limit $ 8.93 $ 8.93 $ 8.93 $ 8.93
Outstanding Option Shares, Range of Exercise Prices, Upper Range Limit 13.32 13.32 13.32 13.32
Options Exercised - Exercise Price 10.55 10.55    
Options Shares Forfeited - Range of Exercise Prices, Lower Range Limit   8.93    
Options Shares Forfeited - Range of Exercise Prices, Upper Range Limit   13.32    
Outstanding Option Shares, Beginning Balance - Weighted Average Exercise Price 11.42 10.78 10.78  
Option Shares Exercised - Weighted Average Exercise Price 10.55 0.00    
Outstanding Option Shares, Ending Balance - Weighted Average Exercise Price $ 10.91 $ 10.78 $ 11.42 $ 10.78