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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Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year.

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-Publisher FASB
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