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Retirement Plans - Expected future benefit payments of US plan (Details) - United States
$ in Thousands
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 1,460
2019 1,700
2020 1,866
2021 2,194
2022 2,336
2023-2027 14,499
Defined benefit plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 1,106
2019 1,248
2020 1,397
2021 1,690
2022 1,800
2023-2027 11,560
Other postretirement benefits plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 354
2019 452
2020 469
2021 504
2022 536
2023-2027 $ 2,939