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Retirement Plans, Expected Employer Contribution and Benefit Payments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Expected employer contributions [Abstract]    
First year $ 0 $ 0
Expected benefit payments (gross) [Abstract]    
Year one 9,483 8,032
Year two 10,131 8,835
Year three 10,818 9,646
Year four 11,608 10,400
Year five 12,377 11,201
Next five years 75,717 70,419
SERP [Member]    
Expected employer contributions [Abstract]    
First year 162 153
Expected benefit payments (gross) [Abstract]    
Year one 162 153
Year two 159 157
Year three 156 155
Year four 152 151
Year five 149 148
Next five years 462 514
Other Postretirement Benefits [Member]    
Expected employer contributions [Abstract]    
First year 48 50
Expected benefit payments (gross) [Abstract]    
Year one 55 58
Year two 56 58
Year three 58 58
Year four 58 58
Year five 46 57
Next five years $ 214 $ 199