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Employee benefit plans - Company's Projected Benefit Payments (Details)
$ in Thousands
Dec. 31, 2019
USD ($)
Retirement Plan  
Defined Benefit Plan Disclosure [Line Items]  
2020 $ 96,448
2021 93,436
2022 94,786
2023 95,830
2024 97,372
2025-2029 508,354
Retiree Health and Life Insurance Plans  
Defined Benefit Plan Disclosure [Line Items]  
2020 1,344
2021 1,305
2022 1,269
2023 1,230
2024 1,173
2025-2029 $ 5,344