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Retirement Benefit Plans (Details)
 3 Months Ended
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Net Periodic Benefit Cost [Line Items]    
Interest credit under cash balance plan 0.03  
Employer matching contribution, percentage 5.00%  
Employees' Pension Plan    
Net Periodic Benefit Cost [Line Items]    
Assumptions used calculating benefit obligation, interest rate to annuitize cash balance account 0.53%  
Defined benefit plan, increase in benefit payable, percent 3.00%  
Defined benefit plan, benefit obligation, increase (decrease) for plan amendment $ 351,638  
Defined benefit plan, amortization of service cost 9 years 8 months 12 days  
Service Cost $ 460,000 $ 430,000
Interest Cost 340,000 417,000
Expected Return on Plan Assets (950,000) (904,000)
Amortization of Prior Service (Credit) Cost 20,000 16,000
Amortization of Net Loss 0 1,000
Net Periodic (Benefit) Cost (130,000) (40,000)
Plan Contributions During the Period $ 0 0
Select Executive Retirement Plan    
Net Periodic Benefit Cost [Line Items]    
Assumptions used calculating benefit obligation, interest rate to annuitize cash balance account 2.31%  
Defined benefit plan, amortization of service cost 12 years 6 months  
Defined benefit plan, benefit obligation, period increase (decrease) $ 122,797  
Service Cost 118,000 103,000
Interest Cost 45,000 54,000
Expected Return on Plan Assets 0 0
Amortization of Prior Service (Credit) Cost 12,000 11,000
Amortization of Net Loss 38,000 33,000
Net Periodic (Benefit) Cost 213,000 201,000
Plan Contributions During the Period $ 118,000 117,000
Post-Retirement Benefit Plans    
Net Periodic Benefit Cost [Line Items]    
Assumptions used calculating benefit obligation, interest rate to annuitize cash balance account 3.09%  
Service Cost $ 31,000 33,000
Interest Cost 67,000 78,000
Expected Return on Plan Assets 0 0
Amortization of Prior Service (Credit) Cost 27,000 27,000
Amortization of Net Loss 8,000 2,000
Net Periodic (Benefit) Cost 133,000 140,000
Plan Contributions During the Period $ 28,000 $ 78,000
On or Subsequent to January 1, 2003    
Net Periodic Benefit Cost [Line Items]    
Service credits, percent of eligible salaries 6.00%  
Minimum | Prior to January 1, 2003    
Net Periodic Benefit Cost [Line Items]    
Service credits, percent of eligible salaries 6.00%  
Maximum | Prior to January 1, 2003    
Net Periodic Benefit Cost [Line Items]    
Service credits, percent of eligible salaries 12.00%