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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Defined Benefit Pension Plans | U.S.        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)        
Service cost $ 223 $ 304 $ 533 $ 609
Interest cost 2,369 2,536 4,741 5,072
Expected return on plan assets (3,176) (3,061) (6,350) (6,122)
Amortization of net loss 0 0 0 0
Curtailment gain recognized (576) 0 (576) 0
Net periodic expense (benefit) (1,160) (221) (1,652) (441)
Defined Benefit Pension Plans | Foreign        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)        
Service cost 845 941 1,732 1,783
Interest cost 1,608 1,254 3,113 2,685
Expected return on plan assets (1,628) (923) (2,947) (2,217)
Amortization of net loss 12 0 25 0
Curtailment gain recognized 0 0 0 0
Net periodic expense (benefit) 837 1,272 1,923 2,251
Supplemental Retirement Plans        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)        
Interest cost 110 123 220 246
Net periodic expense (benefit) 110 123 220 246
Other Postretirement Benefits Plans        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)        
Service cost 3 5 9 10
Interest cost 27 41 65 81
Amortization of prior service credit (20) (20) (39) (39)
Amortization of net loss (11) (19) (22) (38)
Net periodic expense (benefit) $ (1) $ 7 $ 13 $ 14