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Retirement and Benefit Plans (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Pension contributions [Abstract]        
Pension contributions     $ 1,145 $ 2,778
Pension Plan [Member]        
Net periodic benefit cost [Abstract]        
Service cost $ 301 $ 282 602 567
Interest cost 4,382 4,789 8,758 9,571
Expected return on plan assets (4,742) (5,076) (9,482) (10,179)
Amortization of actuarial loss 410 478 805 956
Plan settlement loss 0 0 0 297
Net periodic benefit expense $ 351 $ 473 683 $ 1,212
Pension contributions [Abstract]        
Estimated remaining 2017 pension contributions     $ 1,000