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Retirement Plans, Schedule of Change in Benefit Obligation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year $ 297,325 $ 315,075  
Service cost 13,402 14,683 $ 10,645
Interest cost 14,123 13,302 12,839
Actuarial loss (gain) 19,120 (39,474)  
Gross benefits paid (6,794) (6,261)  
Benefit obligation at end of year 337,176 297,325 315,075
Accumulated benefit obligation at end of year 272,591 238,775  
SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 1,526 1,637  
Service cost 0 0 0
Interest cost 65 64 73
Actuarial loss (gain) 12 (30)  
Gross benefits paid (146) (145)  
Benefit obligation at end of year 1,457 1,526 1,637
Accumulated benefit obligation at end of year 1,457 1,526  
Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 891 1,264  
Service cost 0 0 0
Interest cost 29 36 110
Actuarial loss (gain) (198) (321)  
Gross benefits paid (47) (88)  
Benefit obligation at end of year 675 891 $ 1,264
Accumulated benefit obligation at end of year $ 675 $ 891