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Retirement Plans (Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Net actuarial loss (gain) $ 4,684 $ 12,079 $ (2,438)
Reversal of amortization of net actuarial loss (2,102) (6,462) (7,077)
Retirement Income Plan [Member]      
Service cost 0 1,647 7,215
Interest cost 12,490 12,336 13,668
Expected return on plan assets (19,419) (17,309) (15,969)
Amortization of unrecognized actuarial loss 2,001 6,299 6,831
Total net periodic cost (4,928) 2,973 11,745
Net actuarial loss (gain) 3,594 11,316 (1,425)
Reversal of amortization of net actuarial loss (2,001) (6,299) (6,831)
Total recognized in other comprehensive income 1,593 5,017 (8,256)
Total recognized in net periodic benefit cost and OCI (3,335) 7,990 3,489
Estimated net actuarial loss that will be amortized from AOCI into net periodic benefit cost 2,000    
Retirement Life Plan [Member]      
Total net periodic cost 300    
Supplemental Employee Retirement Plan [Member]      
Total net periodic cost $ 400 $ 500 $ 800