Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Periodic Benefit Cost [Line Items]
Employer matching contribution, Percentage	5.00%
Employees' Pension Plan
Net Periodic Benefit Cost [Line Items]
Service Cost 1	$ 348	$ 350
Interest Cost 2	525	350
Expected Return on Plan Assets 2	(785)	(800)
Amortization of Prior Service (Credit) Cost 2	(12)	(14)
Amortization of Net Loss 2	33	148
Net Periodic Benefit Cost	109	34
Plan Contributions During the Period	0	0
Select Executive Retirement Plan
Net Periodic Benefit Cost [Line Items]
Service Cost 1	11	10
Interest Cost 2	50	50
Expected Return on Plan Assets 2	0	0
Amortization of Prior Service (Credit) Cost 2	14	14
Amortization of Net Loss 2	33	30
Net Periodic Benefit Cost	108	104
Plan Contributions During the Period	116	113
Post-Retirement Benefit Plans
Net Periodic Benefit Cost [Line Items]
Service Cost 1	33	37
Interest Cost 2	99	86
Expected Return on Plan Assets 2	0	0
Amortization of Prior Service (Credit) Cost 2	(3)	(3)
Amortization of Net Loss 2	(6)	0
Net Periodic Benefit Cost	123	120
Plan Contributions During the Period	$ 17	$ 53
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%