Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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	$ 416	$ 382	$ 1,247	$ 1,146
Interest Cost	387	513	1,159	1,253
Expected Return on Plan Assets	(902)	(765)	(2,706)	(2,296)
Amortization of Prior Service (Credit) Cost	15	17	47	52
Amortization of Net Loss	20	153	61	460
Net Periodic (Benefit) Cost	(64)	300	(192)	615
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	102	81	306	243
Interest Cost	48	54	144	162
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	11	14	32	41
Amortization of Net Loss	37	29	110	86
Net Periodic (Benefit) Cost	198	178	592	532
Plan Contributions During the Period	116	117	349	350
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	30	30	92	91
Interest Cost	78	91	233	273
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	27	25	80	76
Amortization of Net Loss	0	(10)	0	(32)
Net Periodic (Benefit) Cost	135	136	405	408
Plan Contributions During the Period	54	$ 51	191	$ 142
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%