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Defined Benefit Post-Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Defined Benefit Post-Retirement Plans [Abstract]          
Employer contributions $ 0 $ 0     $ 0
Pension Benefits [Member]          
Components of net periodic (benefit) cost [Abstract]          
Service cost 420 402 $ 840 $ 801  
Interest cost 920 1,042 1,840 2,084  
Expected return on plan assets (2,123) (1,985) (4,246) (3,970)  
Net amortization 251 415 502 830  
Total net periodic (benefit) cost (532) (126) (1,064) (255)  
Other Benefits [Member]          
Components of net periodic (benefit) cost [Abstract]          
Service cost 3 3 6 6  
Interest cost 82 86 164 172  
Expected return on plan assets 0 0 0 0  
Net amortization 44 20 88 40  
Total net periodic (benefit) cost $ 129 $ 109 $ 258 $ 218