Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Net Periodic Benefit Cost [Line Items]
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			2.04%
Service Cost	$ 430	$ 399
Interest Cost	417	397
Expected Return on Plan Assets	(904)	(770)
Amortization of Prior Service (Credit) Cost	16	17
Amortization of Net Loss	1	153
Net Periodic Cost	(40)	196
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			3.09%
Service Cost	103	97
Interest Cost	54	52
Expected Return on Plan Assets	0	0
Amortization of Prior Service (Credit) Cost	11	14
Amortization of Net Loss	33	27
Net Periodic Cost	201	190
Plan Contributions During the Period	117	116
Post-Retirement Benefit Plans
Net Periodic Benefit Cost [Line Items]
Assumptions used calculating benefit obligation, interest rate to annuitize cash balance account			3.68%
Service Cost	33	30
Interest Cost	78	89
Expected Return on Plan Assets	0	0
Amortization of Prior Service (Credit) Cost	27	25
Amortization of Net Loss	2	(17)
Net Periodic Cost	140	127
Plan Contributions During the Period	$ 78	$ 37
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%