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Employee Benefit Plans - Summary of future benefits payable for Retirement Income Plan over next ten years (Details 8) - Retirement Income Plan
$ in Thousands
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 2,543
2019 2,614
2020 2,659
2021 2,748
2022 2,788
2023-2027 $ 14,490