SUPPLEMENTARY FINANCIAL INFORMATION (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
Supplemental Income Statement Elements [Abstract]
Fee revenue and other income	$ 24,734		$ 24,207	$ 23,800	$ 22,771	$ 22,025	$ 22,149	$ 21,457	$ 20,505	$ 95,512		$ 86,136	$ 83,368
Interest income	10,981		10,665	10,332	9,999	10,158	9,985	10,010	9,777	41,977		39,930	38,710
Interest expense	666		571	470	480	507	505	504	513	2,187		2,029	2,111
Net interest income	10,315		10,094	9,862	9,519	9,651	9,480	9,506	9,264	39,790		37,901	36,599
Provision for loan losses						(500)			(1,000)			(1,500)	(850)
Operating expense	26,142		25,042	24,901	24,318	23,946	23,552	23,059	22,916	100,403		93,473	89,783
Income tax expense	3,576	[1]	2,396	2,248	1,665	1,806	1,855	2,035	2,020	9,885	[1]	7,716	7,978
Net income	$ 5,331		$ 6,863	$ 6,513	$ 6,307	$ 6,424	$ 6,222	$ 5,869	$ 5,833	$ 25,014		$ 24,348	$ 23,056
Net income per share:
Basic earnings per share	$ 0.44		$ 0.56	$ 0.53	$ 0.51	$ .53	$ .51	$ .48	$ .47	$ 2.04		$ 1.99	$ 1.85
Diluted earnings per share	$ 0.43		$ 0.55	$ 0.52	$ 0.51	$ .52	$ .50	$ .47	$ .47	$ 2.01		$ 1.96	$ 1.82

[1] Includes one-time, non-cash TCJA charge of $1,824,000