CREDIT QUALITY ASSESSMENT - Credit Quality of Loan Portfolio, New (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total loans	$ 9,721,348	$ 10,400,509
Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	9,643,181	10,285,057
Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	53,115	112,361
Loans greater than 90 days past due	22,853	774
Restructured loans	2,199	2,317
Total non-performing loans	78,167	115,452
Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	9,593,264	10,226,191
30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	32,281	46,812
60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	17,636	12,054
Commercial investor real estate
Financing Receivable, Past Due [Line Items]
Total loans	3,743,698	3,634,720
Commercial owner-occupied real estate
Financing Receivable, Past Due [Line Items]
Total loans	1,661,092	1,642,216
Commercial AD&C
Financing Receivable, Past Due [Line Items]
Total loans	1,177,949	1,050,973
Residential mortgage
Financing Receivable, Past Due [Line Items]
Total loans	911,997	1,105,179
Residential construction
Financing Receivable, Past Due [Line Items]
Total loans	181,319	182,619
Commercial real estate: | Commercial investor real estate
Financing Receivable, Past Due [Line Items]
Total loans	3,743,698	3,634,720
Commercial real estate: | Commercial investor real estate | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	3,713,482	3,589,360
Commercial real estate: | Commercial investor real estate | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	15,386	45,227
Loans greater than 90 days past due	14,830	133
Restructured loans	0	0
Total non-performing loans	30,216	45,360
Commercial real estate: | Commercial investor real estate | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	3,692,293	3,571,184
Commercial real estate: | Commercial investor real estate | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	10,984	14,046
Commercial real estate: | Commercial investor real estate | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	10,205	4,130
Commercial real estate: | Commercial owner-occupied real estate
Financing Receivable, Past Due [Line Items]
Total loans	1,661,092	1,642,216
Commercial real estate: | Commercial owner-occupied real estate | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,651,238	1,630,655
Commercial real estate: | Commercial owner-occupied real estate | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	9,854	11,561
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	9,854	11,561
Commercial real estate: | Commercial owner-occupied real estate | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,646,076	1,624,265
Commercial real estate: | Commercial owner-occupied real estate | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	0	6,390
Commercial real estate: | Commercial owner-occupied real estate | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	5,162	0
Commercial real estate: | Commercial AD&C
Financing Receivable, Past Due [Line Items]
Total loans	1,177,949	1,050,973
Commercial real estate: | Commercial AD&C | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,169,583	1,035,929
Commercial real estate: | Commercial AD&C | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	1,022	15,044
Loans greater than 90 days past due	7,344	0
Restructured loans	0	0
Total non-performing loans	8,366	15,044
Commercial real estate: | Commercial AD&C | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,169,583	1,033,057
Commercial real estate: | Commercial AD&C | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	0	29
Commercial real estate: | Commercial AD&C | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	0	2,843
Commercial Business
Financing Receivable, Past Due [Line Items]
Total loans	8,177,267	8,595,457
Commercial Business | Commercial business
Financing Receivable, Past Due [Line Items]
Total loans	1,594,528	2,267,548
Commercial Business | Commercial business | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,584,440	2,243,739
Commercial Business | Commercial business | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	9,454	22,933
Loans greater than 90 days past due	0	161
Restructured loans	634	715
Total non-performing loans	10,088	23,809
Commercial Business | Commercial business | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	1,579,499	2,238,617
Commercial Business | Commercial business | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	4,461	4,859
Commercial Business | Commercial business | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	480	263
Residential real estate: | Residential mortgage
Financing Receivable, Past Due [Line Items]
Total loans	911,997	1,105,179
Residential real estate: | Residential mortgage | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	900,242	1,092,885
Residential real estate: | Residential mortgage | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	9,511	10,212
Loans greater than 90 days past due	679	480
Restructured loans	1,565	1,602
Total non-performing loans	11,755	12,294
Residential real estate: | Residential mortgage | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	888,345	1,073,963
Residential real estate: | Residential mortgage | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	11,501	16,213
Residential real estate: | Residential mortgage | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	396	2,709
Residential real estate: | Residential construction
Financing Receivable, Past Due [Line Items]
Total loans	181,319	182,619
Residential real estate: | Residential construction | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	181,257	182,619
Residential real estate: | Residential construction | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	62	0
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	62	0
Residential real estate: | Residential construction | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	180,496	182,557
Residential real estate: | Residential construction | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	761	0
Residential real estate: | Residential construction | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	0	62
Consumer
Financing Receivable, Past Due [Line Items]
Total loans	450,765	517,254
Consumer | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	442,939	509,870
Consumer | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	7,826	7,384
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	7,826	7,384
Consumer | Current | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	436,972	502,548
Consumer | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	4,574	5,275
Consumer | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Total loans	$ 1,393	$ 2,047