Quarterly Unaudited Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Unaudited Financial Data [Abstract]
Total Interest Income	$ 29,692	$ 29,609	$ 28,069	$ 27,242	$ 25,664	$ 25,262	$ 24,314	$ 24,026	$ 114,612	$ 99,266	$ 90,075
Total Interest Expense	1,616	1,759	1,538	1,376	1,215	1,110	951	920	6,289	4,196	3,325
Net Interest Income	28,076	27,850	26,531	25,866	24,449	24,152	23,363	23,106	108,323	95,070	86,750
Provision for Credit Losses	0	1,600	650	600	3,485	250	0	2,600	2,850	6,335	750
Net Interest Income After Provision for Credit Losses	28,076	26,250	25,881	25,266	20,964	23,902	23,363	20,506	105,473	88,735	86,000
Total Non-Interest Income	4,179	3,638	3,539	5,406	5,108	4,553	2,875	2,721	16,762	15,257	14,575
Total Non-Interest Expense	16,500	16,307	16,525	18,422	15,002	16,414	14,737	12,019	67,754	58,172	56,259
Income Before Income Taxes	15,755	13,581	12,895	12,250	11,070	12,041	11,501	11,208	54,481	45,820	44,316
Provision for Income Taxes	11,974	5,000	4,708	4,429	3,389	4,503	4,169	4,036	26,111	16,097	16,924
Net Income	$ 3,781	$ 8,581	$ 8,187	$ 7,821	$ 7,681	$ 7,538	$ 7,332	$ 7,172	$ 28,370	$ 29,723	$ 27,392
Basic Earnings Per Common Share (in dollars per share)	$ 4.64	$ 10.59	$ 10.12	$ 9.68	$ 9.62	$ 9.51	$ 9.25	$ 9.06	$ 35.03	$ 37.44	$ 34.82