Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Periodic Benefit Cost [Line Items]
Employer matching contribution, Percentage			5.00%
Employees' Pension Plan
Net Periodic Benefit Cost [Line Items]
Service Cost	$ 431	$ 350	$ 779	$ 700
Interest Cost	274	373	799	723
Expected Return on Plan Assets	(896)	(800)	(1,681)	(1,600)
Amortization of Prior Service (Credit) Cost	(13)	(14)	(25)	(28)
Amortization of Net Loss	64	148	97	296
Net Periodic (Benefit) Cost	(140)	57	(31)	91
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]
Service Cost	196	10	207	20
Interest Cost	54	59	104	109
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	15	14	29	28
Amortization of Net Loss	33	33	66	63
Net Periodic (Benefit) Cost	298	116	406	220
Plan Contributions During the Period	117	116	233	229
Estimated Future Contributions in the Current Fiscal Year	0		0
Post-Retirement Benefit Plans
Net Periodic Benefit Cost [Line Items]
Service Cost	35	37	68	74
Interest Cost	68	63	167	149
Expected Return on Plan Assets	0	0	0	0
Amortization of Prior Service (Credit) Cost	53	(3)	50	(6)
Amortization of Net Loss	6	0	0	0
Net Periodic (Benefit) Cost	162	97	285	217
Plan Contributions During the Period	102	$ 177	119	$ 230
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%