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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest income	$ 85,390	$ 87,082	$ 87,214	$ 88,183	$ 85,607	$ 84,374	$ 78,597	$ 75,504	$ 347,869	$ 324,082	$ 194,799
Interest expense	19,807	20,292	21,029	21,433	19,462	16,783	14,779	12,613	82,561	63,637	26,031
Provision for loan losses	1,655	1,524	1,633	(128)	3,403	1,890	1,733	1,997	4,684	9,023	2,977
Non-interest income	19,224	18,573	16,556	16,969	14,030	15,033	14,868	17,118	71,322	61,049	51,243
Non-interest Expense	46,081	44,925	43,887	44,192	42,667	42,393	45,082	49,641	179,085	179,783	129,099
Income before income taxes	37,071	38,914	37,221	39,655	34,105	38,341	31,871	28,371	152,861	132,688	87,935
Income tax expense	8,614	9,531	8,945	9,338	8,539	9,107	7,472	6,706	36,428	31,824	34,726
Net income	28,457	$ 29,383	$ 28,276	$ 30,317	25,566	$ 29,234	$ 24,399	$ 21,665	116,433	100,864	53,209
Assets	8,629,002				8,243,272				8,629,002	8,243,272	5,446,675
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income									(37)	(10)	(8)
Interest expense									(37)	(10)	(8)
Provision for loan losses									0	0	0
Non-interest income									(667)	(615)	(772)
Non-interest Expense									(667)	(615)	(772)
Income before income taxes									0	0	0
Income tax expense									0	0	0
Net income									0	0	0
Assets	(22,352)				(28,507)				(22,352)	(28,507)	(21,380)
Community Banking
Segment Reporting Information [Line Items]
Interest income									347,867	324,081	194,798
Interest expense									82,598	63,647	26,039
Provision for loan losses									4,684	9,023	2,977
Non-interest income									55,042	45,841	37,447
Non-interest Expense									166,802	168,261	119,607
Income before income taxes									148,825	128,991	83,622
Income tax expense									35,350	30,827	33,684
Net income									113,475	98,164	49,938
Assets	8,624,590				8,246,282				8,624,590	8,246,282	5,446,056
Insurance
Segment Reporting Information [Line Items]
Interest income									26	3	2
Interest expense									0	0	0
Provision for loan losses									0	0	0
Non-interest income									6,621	6,153	6,233
Non-interest Expense									5,731	5,601	5,533
Income before income taxes									916	555	702
Income tax expense									258	169	(399)
Net income									658	386	1,101
Assets	10,340				9,165				10,340	9,165	8,873
Investment Management
Segment Reporting Information [Line Items]
Interest income									13	8	7
Interest expense									0	0	0
Provision for loan losses									0	0	0
Non-interest income									10,326	9,670	8,335
Non-interest Expense									7,219	6,536	4,731
Income before income taxes									3,120	3,142	3,611
Income tax expense									820	828	1,441
Net income									2,300	2,314	2,170
Assets	$ 16,424				$ 16,332				$ 16,424	$ 16,332	$ 13,126