SEGMENT REPORTING (Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements) (Detail) - USD ($) $ 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
Segment Reporting Information [Line Items]
Interest income	$ 50,676	$ 49,589	$ 48,576	$ 45,958	$ 44,243	$ 42,857	$ 41,803	$ 41,653	$ 194,799	$ 170,556	$ 158,312
Interest expense	7,184	6,892	6,250	5,705	5,276	5,126	5,071	5,531	26,031	21,004	20,113
Provision for loan losses	527	934	1,322	194	572	781	2,957	1,236	2,977	5,546	5,371
Non-interest income	12,294	12,746	13,571	12,632	12,344	12,584	12,751	13,363	51,243	51,042	49,901
Non-interest Expense	35,059	31,191	32,868	29,981	30,544	29,326	30,871	32,317	129,099	123,058	115,347
Income before income taxes	20,200	23,318	21,707	22,710	20,195	20,208	15,655	15,932	87,935	71,990	67,382
Income tax expense	11,933	8,229	6,966	7,598	6,879	6,734	5,008	5,119	34,726	23,740	22,027
Net income	8,267	$ 15,089	$ 14,741	$ 15,112	13,316	$ 13,474	$ 10,647	$ 10,813	53,209	48,250	45,355
Assets	5,446,675				5,091,383				5,446,675	5,091,383	4,655,380
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income									(8)	(8)	(6)
Interest expense									(8)	(8)	(6)
Provision for loan losses									0	0	0
Non-interest income									(772)	(713)	(16,117)
Non-interest Expense									(772)	(713)	(16,117)
Income before income taxes									0	0	0
Income tax expense									0	0	0
Net income									0	0	0
Assets	(21,380)				(22,282)				(21,380)	(22,282)	(19,393)
Community Banking
Segment Reporting Information [Line Items]
Interest income									194,798	170,556	158,313
Interest expense									26,039	21,012	20,119
Provision for loan losses									2,977	5,546	5,371
Non-interest income									37,447	38,769	53,398
Non-interest Expense									119,607	114,368	122,183
Income before income taxes									83,622	68,399	64,038
Income tax expense									33,684	22,337	20,710
Net income									49,938	46,062	43,328
Assets	5,446,056				5,092,283				5,446,056	5,092,283	4,656,573
Insurance
Segment Reporting Information [Line Items]
Interest income									2	3	1
Interest expense									0	0	0
Provision for loan losses									0	0	0
Non-interest income									6,233	5,418	5,516
Non-interest Expense									5,533	5,097	5,189
Income before income taxes									702	324	328
Income tax expense									(399)	130	141
Net income									1,101	194	187
Assets	8,873				7,732				8,873	7,732	5,542
Investment Management
Segment Reporting Information [Line Items]
Interest income									7	5	4
Interest expense									0	0	0
Provision for loan losses									0	0	0
Non-interest income									8,335	7,568	7,104
Non-interest Expense									4,731	4,306	4,092
Income before income taxes									3,611	3,267	3,016
Income tax expense									1,441	1,273	1,176
Net income									2,170	1,994	1,840
Assets	$ 13,126				$ 13,650				$ 13,126	$ 13,650	$ 12,658