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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Interest income	$ 111,129	$ 112,984	$ 340,213	$ 311,769
Interest expense	4,525	15,500	20,963	48,437
Provision/ (credit) for credit losses	(8,229)	7,003	(47,141)	90,158	$ 85,669
Non-interest income	24,394	29,390	79,519	70,482
Non-interest expense	63,181	60,937	194,329	194,121
Income before income tax expense	76,046	58,934	251,581	49,535
Income tax expense	19,070	14,292	61,878	9,244
Net income	56,976	44,642	189,703	40,291
Assets	13,017,464	12,678,131	13,017,464	12,678,131	$ 12,798,429
Inter-Segment Elimination
Segment Reporting Information [Line Items]
Interest income	(3)	(3)	(9)	(10)
Interest expense	(3)	(3)	(9)	(10)
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	(217)	(221)	(4,651)	(645)
Non-interest expense	(217)	(221)	(651)	(645)
Income before income tax expense	0	0	(4,000)	0
Income tax expense	0	0	0	0
Net income	0	0	(4,000)	0
Assets	(66,527)	(68,642)	(66,527)	(68,642)
Community Banking | Operating Segments
Segment Reporting Information [Line Items]
Interest income	111,129	112,985	340,213	311,771
Interest expense	4,528	15,503	20,972	48,447
Provision/ (credit) for credit losses	(8,229)	7,003	(47,141)	90,158
Non-interest income	16,644	24,746	62,083	57,976
Non-interest expense	58,059	57,856	179,803	184,921
Income before income tax expense	73,415	57,369	248,662	46,221
Income tax expense	18,385	13,912	59,951	8,396
Net income	55,030	43,457	188,711	37,825
Assets	13,017,464	12,678,396	13,017,464	12,678,396
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Interest income	0	1	2	5
Interest expense	0	0	0	0
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	2,282	2,119	5,679	5,446
Non-interest expense	1,696	1,580	4,507	4,319
Income before income tax expense	586	540	1,174	1,132
Income tax expense	185	149	347	313
Net income	401	391	827	819
Assets	9,203	11,289	9,203	11,289
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Interest income	3	1	7	3
Interest expense	0	0	0	0
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	5,685	2,746	16,408	7,705
Non-interest expense	3,643	1,722	10,670	5,526
Income before income tax expense	2,045	1,025	5,745	2,182
Income tax expense	500	231	1,580	535
Net income	1,545	794	4,165	1,647
Assets	$ 57,324	$ 57,088	$ 57,324	$ 57,088