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Retirement Plans (Components of Net Periodic Benefit Cost) (Details) - Pension Plan [Member] - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost $ 3,095 $ 3,111
Expected return on plan assets (5,682) (4,854)
Amortization of unrecognized net actuarial loss 494 481
Total net periodic benefit $ (2,093) $ (1,262)