SEGMENT REPORTING - Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Interest income	$ 114,823	$ 114,927	$ 229,084	$ 198,785
Interest expense	6,777	13,413	16,438	32,937
Provision/ (credit) for credit losses	(4,204)	58,686	(38,912)	83,155	$ 85,669
Non-interest income	26,259	22,924	55,125	41,092
Non-interest expense	62,975	85,438	131,148	133,184
Income/ (loss) before income tax expense/ (benefit)	75,534	(19,686)	175,535	(9,399)
Income tax expense/ (benefit)	18,271	(5,348)	42,808	(5,048)
Net income/ (loss)	57,263	(14,338)	132,727	(4,351)
Assets	12,925,577	13,290,447	12,925,577	13,290,447	$ 12,798,429
Inter-Segment Elimination
Segment Reporting Information [Line Items]
Interest income	(3)	(3)	(6)	(7)
Interest expense	(3)	(3)	(6)	(7)
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	(4,217)	(220)	(4,434)	(424)
Non-interest expense	(217)	(220)	(434)	(424)
Income/ (loss) before income tax expense/ (benefit)	(4,000)	0	(4,000)	0
Income tax expense/ (benefit)	0	0	0	0
Net income/ (loss)	(4,000)	0	(4,000)	0
Assets	(64,406)	(56,892)	(64,406)	(56,892)
Community Banking | Operating Segments
Segment Reporting Information [Line Items]
Interest income	114,823	114,929	229,084	198,786
Interest expense	6,780	13,416	16,444	32,944
Provision/ (credit) for credit losses	(4,204)	58,686	(38,912)	83,155
Non-interest income	23,714	19,253	45,439	33,230
Non-interest expense	58,049	82,410	121,744	127,065
Income/ (loss) before income tax expense/ (benefit)	77,912	(20,330)	175,247	(11,148)
Income tax expense/ (benefit)	17,728	(5,517)	41,566	(5,516)
Net income/ (loss)	60,184	(14,813)	133,681	(5,632)
Assets	12,925,577	13,281,483	12,925,577	13,281,483
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Interest income	1	1	2	4
Interest expense	0	0	0	0
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	1,246	1,198	3,397	3,327
Non-interest expense	1,324	1,275	2,811	2,739
Income/ (loss) before income tax expense/ (benefit)	(77)	(76)	588	592
Income tax expense/ (benefit)	(21)	(21)	162	164
Net income/ (loss)	(56)	(55)	426	428
Assets	8,758	10,537	8,758	10,537
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Interest income	2	0	4	2
Interest expense	0	0	0	0
Provision/ (credit) for credit losses	0	0	0	0
Non-interest income	5,516	2,693	10,723	4,959
Non-interest expense	3,819	1,973	7,027	3,804
Income/ (loss) before income tax expense/ (benefit)	1,699	720	3,700	1,157
Income tax expense/ (benefit)	564	190	1,080	304
Net income/ (loss)	1,135	530	2,620	853
Assets	$ 55,648	$ 55,319	$ 55,648	$ 55,319