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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Defined benefit pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1,108 $ 1,077 $ 2,192 $ 2,243
Interest cost 4,555 3,571 9,197 8,891
Expected return on plan assets (5,657) (5,752) (11,331) (11,485)
Amortization of prior service cost 8 7 16 15
Recognized actuarial (gain) loss 2,126 1,821 4,523 3,641
Settlement loss 2,401 0 2,443 0
Net periodic benefit cost 4,541 724 7,040 3,305
Postretirement benefit plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0 0 1 1
Interest cost 52 151 104 406
Amortization of prior service cost (509) 0 (1,019) 0
Recognized actuarial (gain) loss (80) 38 (160) 75
Net periodic benefit cost $ (537) $ 189 $ (1,074) $ 482