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Retirement Plans, Schedule of Change in Benefit Obligation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year $ 315,075 $ 249,960  
Service cost 14,683 10,645 $ 12,824
Interest cost 13,302 12,839 11,400
Actuarial loss (gain) (39,474) 66,640  
Gross benefits paid (6,261) (25,009)  
Benefit obligation at end of year 297,325 315,075 249,960
Accumulated benefit obligation at end of year 238,775 241,592  
SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 1,637 1,529  
Service cost 0 0 0
Interest cost 64 73 70
Actuarial loss (gain) (30) 180  
Gross benefits paid (145) (145)  
Benefit obligation at end of year 1,526 1,637 1,529
Accumulated benefit obligation at end of year 1,526 1,637  
Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 1,264 2,307  
Service cost 0 0 0
Interest cost 36 110 112
Actuarial loss (gain) (321) (1,065)  
Gross benefits paid (88) (88)  
Benefit obligation at end of year 891 1,264 $ 2,307
Accumulated benefit obligation at end of year $ 0 $ 0