Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 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	$ 401	$ 365	$ 831	$ 764
Interest Cost	355	343	772	740
Expected Return on Plan Assets	(900)	(761)	(1,804)	(1,531)
Amortization of Prior Service (Credit) Cost	16	18	32	35
Amortization of Net Loss	40	154	41	307
Net Periodic Cost	(88)	119	(128)	315
Plan Contributions During the Period	0	0	0	0
Estimated Future Contributions in the Current Fiscal Year	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	101	65	204	162
Interest Cost	42	56	96	108
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	10	13	21	27
Amortization of Net Loss	40	30	73	57
Net Periodic Cost	193	164	394	354
Plan Contributions During the Period	116	117	233	233
Estimated Future Contributions in the Current Fiscal Year	0		0
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	29	31	62	61
Interest Cost	77	93	155	182
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	26	26	53	51
Amortization of Net Loss	(2)	(5)	0	(22)
Net Periodic Cost	130	145	270	272
Plan Contributions During the Period	59	$ 54	137	$ 91
Estimated Future Contributions in the Current Fiscal Year	$ 0		$ 0
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%