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Estimated future benefit payments and subsidies (Details 8) - Qualified Plan - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2020
Underfunded Plan | Pension Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  | $ 0
Defined Benefit Plan, Plan Assets, Additional Contributions by Employer | $ 20,000,000.0 |
2021 |  | 24,455,000
2022 |  | 24,507,000
2023 |  | 24,733,000
2024 |  | 24,835,000
2025 |  | 24,713,000
2026-2030 |  | 119,191,000
Overfunded Plan | Other Postretirement Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  | 500,000
2021 |  | 5,404,000
2022 |  | 5,437,000
2023 |  | 5,424,000
2024 |  | 5,377,000
2025 |  | 5,277,000
2026-2030 |  | 25,949,000
2021 |  | 95,000
2022 |  | 89,000
2023 |  | 83,000
2024 |  | 75,000
2025 |  | 70,000
2026-2030 |  | $ 252,000

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