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Retirement Plans, Projected Benefit Obligation in Excess of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Projected benefit obligation at end of year $ 337,176 $ 297,325
Fair value of plan assets at end of year 257,517 243,588
SERP [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Projected benefit obligation at end of year 1,457 1,526
Fair value of plan assets at end of year $ 0 $ 0