SEGMENT REPORTING (Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest income	$ 87,082	$ 84,374	$ 262,479	$ 238,475
Interest expense	20,292	16,783	62,754	44,175
Provision (credit) for loan losses	1,524	1,890	3,029	5,620	$ 9,023
Non-interest income	18,573	15,033	52,098	47,019
Non-interest Expense	44,925	42,393	133,004	137,116
Income before income taxes	38,914	38,341	115,790	98,583
Income tax expense	9,531	9,107	27,814	23,285
Net income	29,383	29,234	87,976	75,298
Assets	8,437,538	8,034,565	8,437,538	8,034,565	$ 8,243,272
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(3)	(3)	(10)	(6)
Interest expense	(3)	(3)	(10)	(6)
Provision (credit) for loan losses	0	0	0	0
Non-interest income	(168)	(154)	(501)	(461)
Non-interest Expense	(168)	(154)	(501)	(461)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	(33,698)	(29,140)	(33,698)	(29,140)
Community Banking
Segment Reporting Information [Line Items]
Interest income	87,068	84,374	262,465	238,474
Interest expense	20,295	16,786	62,764	44,181
Provision (credit) for loan losses	1,524	1,890	3,029	5,620
Non-interest income	14,080	10,707	39,643	35,130
Non-interest Expense	41,806	39,372	123,876	128,705
Income before income taxes	37,523	37,033	112,439	95,098
Income tax expense	9,162	8,755	26,924	22,357
Net income	28,361	28,278	85,515	72,741
Assets	8,437,588	8,038,263	8,437,588	8,038,263
Insurance
Segment Reporting Information [Line Items]
Interest income	13	1	15	2
Interest expense	0	0	0	0
Provision (credit) for loan losses	0	0	0	0
Non-interest income	2,123	2,018	5,296	5,019
Non-interest Expense	1,635	1,564	4,359	4,245
Income before income taxes	501	455	952	776
Income tax expense	138	127	265	218
Net income	363	328	687	558
Assets	11,615	9,473	11,615	9,473
Investment Management
Segment Reporting Information [Line Items]
Interest income	4	2	9	5
Interest expense	0	0	0	0
Provision (credit) for loan losses	0	0	0	0
Non-interest income	2,538	2,462	7,660	7,331
Non-interest Expense	1,652	1,611	5,270	4,627
Income before income taxes	890	853	2,399	2,709
Income tax expense	231	225	625	710
Net income	659	628	1,774	1,999
Assets	$ 22,033	$ 15,969	$ 22,033	$ 15,969