Pension Plans and Other Postretirement Benefits (Details-6) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expected Cash Flows
Expected employer contributions, 2021	$ 0
Cash contributions to pension and other postretirement plans	6,296,000	$ 5,714,000	$ 5,584,000
Pension Benefits [Member]
Benefit Payments, Gross
Defined benefit plan expected future benefit payments, 2021	18,493,000
Defined benefit plan expected future benefit payments, 2022	19,508,000
Defined benefit plan expected future benefit payments, 2023	20,353,000
Defined benefit plan expected future benefit payments, 2024	21,007,000
Defined benefit plan expected future benefit payments, 2025	21,678,000
Defined benefit plan expected future benefit payments, 2026-2030	114,762,000
Postretirement Benefits [Member]
Benefit Payments, Gross
Defined benefit plan expected future benefit payments, 2021	4,696,000
Defined benefit plan expected future benefit payments, 2022	4,859,000
Defined benefit plan expected future benefit payments, 2023	5,145,000
Defined benefit plan expected future benefit payments, 2024	5,443,000
Defined benefit plan expected future benefit payments, 2025	5,529,000
Defined benefit plan expected future benefit payments, 2026-2030	27,163,000
Medicare Part D Subsidies
Expected Medicare Part D Subsidies, 2021	(250,000)
Expected Medicare Part D Subsidies, 2022	(277,000)
Expected Medicare Part D Subsidies, 2023	(302,000)
Expected Medicare Part D Subsidies, 2024	(327,000)
Expected Medicare Part D Subsidies, 2025	(354,000)
Expected Medicare Part D Subsidies, 2026-2030	(2,148,000)
Benefit Payments, Net
Defined benefit plan expected net future benefit payments, 2021	4,446,000
Defined benefit plan expected net future benefit payments, 2022	4,582,000
Defined benefit plan expected net future benefit payments, 2023	4,843,000
Defined benefit plan expected net future benefit payments, 2024	5,116,000
Defined benefit plan expected net future benefit payments, 2025	5,175,000
Defined benefit plan expected net future benefit payments, 2026-2030	$ 25,015,000