Other Comprehensive Income (Loss) (Schedule of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense									$ (9,528)	$ (11,534)	$ (13,946)
Net Income	$ 10,980	$ 12,639	$ 11,097	$ 11,813	$ 11,621	$ 9,660	$ 9,839	$ 9,556	46,529	40,676	35,184
Salaries and Employee Benefits									(51,306)	(46,642)	(43,961)
Total Reclassifications for the Period									535	382
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total Reclassifications for the Period									558	387
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total Reclassifications for the Period									535	382	1,282
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Gain (Loss) on Securities									706	596	1,979
Income Tax Expense									(148)	(209)	(693)
Net Income									558	387	1,286
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Post Retirement Plan Items Actuarial Gains (Losses) | Postretirement Medical and Life Benefit Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense									9	3	2
Net Income									(23)	(5)	(4)
Salaries and Employee Benefits									$ (32)	$ (8)	$ (6)