Quarterly report pursuant to Section 13 or 15(d)

Benefit Plans - Net Periodic Pension Expense Benefit (Details)

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Benefit Plans - Net Periodic Pension Expense Benefit (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Pension Plans | U.S.    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)    
Service cost $ 310 $ 305
Interest cost 2,372 2,536
Expected return on plan assets (3,174) (3,061)
Amortization of net loss 0 0
Net periodic expense (benefit) (492) (220)
Defined Benefit Pension Plans | Foreign    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)    
Service cost 887 842
Interest cost 1,505 1,431
Expected return on plan assets (1,319) (1,294)
Amortization of net loss 13 0
Net periodic expense (benefit) 1,086 979
Supplemental Retirement Plans    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)    
Interest cost 110 123
Net periodic expense (benefit) 110 123
Other Postretirement Benefits Plans    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)    
Service cost 6 5
Interest cost 38 40
Amortization of prior service credit (19) (19)
Amortization of net loss (11) (19)
Net periodic expense (benefit) $ 14 $ 7