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Employee Benefit Plans - Schedule of expected benefit payments (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Defined Benefit Pension Plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 20,286
2019 21,396
2020 22,737
2021 24,688
2022 26,676
2023 through 2027 158,881
Post-Retirement Medical Plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 795
2019 855
2020 873
2021 940
2022 1,003
2023 through 2027 6,579
Supplemental Executive Retirement Plan  
Defined Benefit Plan Disclosure [Line Items]  
2018 2,515
2019 2,520
2020 2,480
2021 2,446
2022 10,180
2023 through 2027 $ 13,065