CREDIT QUALITY ASSESSMENT - Credit Quality of Loan Portfolio, New (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total	$ 10,446,866	$ 10,400,509
Performing loans:
Financing Receivable, Past Due [Line Items]
Current	10,296,660	10,226,191
Total performing loans	10,348,166	10,285,057
Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	96,000	112,361
Loans greater than 90 days past due	429	774
Restructured loans	2,271	2,317
Total non-performing loans	98,700	115,452
30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	41,925	46,812
60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	9,581	12,054
Commercial investor real estate
Financing Receivable, Past Due [Line Items]
Total	3,652,418	3,634,720
Commercial owner-occupied real estate
Financing Receivable, Past Due [Line Items]
Total	1,644,848	1,642,216
Commercial AD&C
Financing Receivable, Past Due [Line Items]
Total	1,051,013	1,050,973
Residential mortgage
Financing Receivable, Past Due [Line Items]
Total	1,022,546	1,105,179
Residential construction
Financing Receivable, Past Due [Line Items]
Total	171,028	182,619
Commercial real estate: | Commercial investor real estate
Financing Receivable, Past Due [Line Items]
Total	3,652,418	3,634,720
Commercial real estate: | Commercial investor real estate | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	3,587,859	3,571,184
Total performing loans	3,609,642	3,589,360
Commercial real estate: | Commercial investor real estate | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	42,776	45,227
Loans greater than 90 days past due	0	133
Restructured loans	0	0
Total non-performing loans	42,776	45,360
Commercial real estate: | Commercial investor real estate | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	17,870	14,046
Commercial real estate: | Commercial investor real estate | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	3,913	4,130
Commercial real estate: | Commercial owner-occupied real estate
Financing Receivable, Past Due [Line Items]
Total	1,644,848	1,642,216
Commercial real estate: | Commercial owner-occupied real estate | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	1,633,446	1,624,265
Total performing loans	1,636,532	1,630,655
Commercial real estate: | Commercial owner-occupied real estate | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	8,316	11,561
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	8,316	11,561
Commercial real estate: | Commercial owner-occupied real estate | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	3,086	6,390
Commercial real estate: | Commercial owner-occupied real estate | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	0	0
Commercial real estate: | Commercial AD&C
Financing Receivable, Past Due [Line Items]
Total	1,051,013	1,050,973
Commercial real estate: | Commercial AD&C | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	1,031,055	1,033,057
Total performing loans	1,036,038	1,035,929
Commercial real estate: | Commercial AD&C | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	14,975	15,044
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	14,975	15,044
Commercial real estate: | Commercial AD&C | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	4,983	29
Commercial real estate: | Commercial AD&C | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	0	2,843
Commercial Business
Financing Receivable, Past Due [Line Items]
Total	8,759,388	8,595,457
Commercial Business | Commercial business
Financing Receivable, Past Due [Line Items]
Total	2,411,109	2,267,548
Commercial Business | Commercial business | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	2,392,818	2,238,617
Total performing loans	2,397,247	2,243,739
Commercial Business | Commercial business | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	13,147	22,933
Loans greater than 90 days past due	31	161
Restructured loans	684	715
Total non-performing loans	13,862	23,809
Commercial Business | Commercial business | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	3,983	4,859
Commercial Business | Commercial business | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	446	263
Residential real estate: | Residential mortgage
Financing Receivable, Past Due [Line Items]
Total	1,022,546	1,105,179
Residential real estate: | Residential mortgage | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	997,837	1,073,963
Total performing loans	1,010,968	1,092,885
Residential real estate: | Residential mortgage | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	9,593	10,212
Loans greater than 90 days past due	398	480
Restructured loans	1,587	1,602
Total non-performing loans	11,578	12,294
Residential real estate: | Residential mortgage | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	9,593	16,213
Residential real estate: | Residential mortgage | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	3,538	2,709
Residential real estate: | Residential construction
Financing Receivable, Past Due [Line Items]
Total	171,028	182,619
Residential real estate: | Residential construction | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	170,365	182,557
Total performing loans	171,028	182,619
Residential real estate: | Residential construction | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	0	0
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	0	0
Residential real estate: | Residential construction | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	601	0
Residential real estate: | Residential construction | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	62	62
Consumer
Financing Receivable, Past Due [Line Items]
Total	493,904	517,254
Consumer | Performing loans:
Financing Receivable, Past Due [Line Items]
Current	483,280	502,548
Total performing loans	486,711	509,870
Consumer | Non-performing loans:
Financing Receivable, Past Due [Line Items]
Non-accrual loans	7,193	7,384
Loans greater than 90 days past due	0	0
Restructured loans	0	0
Total non-performing loans	7,193	7,384
Consumer | 30-59 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	1,809	5,275
Consumer | 60-89 days | Performing loans:
Financing Receivable, Past Due [Line Items]
Past due	$ 1,622	$ 2,047