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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 112,984	$ 87,082	$ 311,769	$ 262,479
Interest expense	15,500	20,292	48,437	62,754
Provision for credit losses	7,003	1,524	90,158	3,029	$ 4,684
Non-interest income	29,390	18,573	70,482	52,098
Non-interest expense	60,937	44,925	194,121	133,004
Income before income taxes	58,934	38,914	49,535	115,790
Income tax expense	14,292	9,531	9,244	27,814
Net income	44,642	29,383	40,291	87,976
Assets	12,678,131	8,437,538	12,678,131	8,437,538	$ 8,629,002
Inter-Segment Elimination
Segment Reporting Information [Line Items]
Interest income	(3)	(3)	(10)	(10)
Interest expense	(3)	(3)	(10)	(10)
Provision for credit losses	0	0	0	0
Non-interest income	(221)	(168)	(645)	(501)
Non-interest expense	(221)	(168)	(645)	(501)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	(68,642)	(33,698)	(68,642)	(33,698)
Community Banking | Operating Segments
Segment Reporting Information [Line Items]
Interest income	112,985	87,068	311,771	262,465
Interest expense	15,503	20,295	48,447	62,764
Provision for credit losses	7,003	1,524	90,158	3,029
Non-interest income	24,746	14,080	57,976	39,643
Non-interest expense	57,856	41,806	184,921	123,876
Income before income taxes	57,369	37,523	46,221	112,439
Income tax expense	13,912	9,162	8,396	26,924
Net income	43,457	28,361	37,825	85,515
Assets	12,678,396	8,437,588	12,678,396	8,437,588
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Interest income	1	13	5	15
Interest expense	0	0	0	0
Provision for credit losses	0	0	0	0
Non-interest income	2,119	2,123	5,446	5,296
Non-interest expense	1,580	1,635	4,319	4,359
Income before income taxes	540	501	1,132	952
Income tax expense	149	138	313	265
Net income	391	363	819	687
Assets	11,289	11,615	11,289	11,615
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Interest income	1	4	3	9
Interest expense	0	0	0	0
Provision for credit losses	0	0	0	0
Non-interest income	2,746	2,538	7,705	7,660
Non-interest expense	1,722	1,652	5,526	5,270
Income before income taxes	1,025	890	2,182	2,399
Income tax expense	231	231	535	625
Net income	794	659	1,647	1,774
Assets	$ 57,088	$ 22,033	$ 57,088	$ 22,033