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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined benefit pension plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 1,084 $ 1,166
Interest cost 4,642 5,320
Expected return on plan assets (5,674) (5,733)
Amortization of prior service cost 8 8
Recognized actuarial (gain) loss 2,397 1,820
Settlement loss 42 0
Net periodic benefit cost 2,499 2,581
Postretirement benefit plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0 1
Interest cost 52 255
Amortization of prior service cost (509) 0
Recognized actuarial (gain) loss (80) 37
Net periodic benefit cost $ (537) $ 293