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Post Retirement Benefits - Post Retirement Benefits Expense (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension, health and life insurance expense:      
Multi-employer plan $ 676,000 $ 971,000 $ 760,000
Defined contribution plans 1,173,000 1,258,000 1,059,000
Total pension expense 1,849,000 2,229,000 1,819,000
Interest cost 235,000 285,000 277,000
Amortization of prior service costs (496,000) (496,000) (496,000)
Amortization of net loss 181,000 117,000 171,000
Net periodic benefit cost (80,000) (94,000) (48,000)
Total post retirement benefits expense $ 1,769,000 $ 2,135,000 $ 1,771,000