ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 47,959	$ 44,846
Accruing Current Balances	8,489,391	8,927,923
Nonaccrual Loans	31,792	41,908
Total Loans	$ 8,569,142	$ 9,014,677	$ 8,534,986
Percent past due	0.56%	0.50%
Percent of Accruing Current Balances	99.07%	99.04%
Percent of nonaccrual loans	0.37%	0.46%
% of Total Loans	100.00%	100.00%
Nonaccrual with an allowance	$ 11	$ 13
Reverse mortgage, fair value	9,400	10,100	15,800
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 40,281	$ 28,152
Percent past due	0.47%	0.31%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 7,678	$ 16,694
Percent past due	0.09%	0.19%
Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 1,717	$ 3,074
Accruing Current Balances	666,690	758,072
Nonaccrual Loans	3,173	3,247
Total Loans	671,580	764,393	955,765
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	967	3,049
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	750	25
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	17,110	19,390
Accruing Current Balances	1,120,608	1,144,217
Nonaccrual Loans	2,316	2,310
Total Loans	1,140,034	1,165,917	1,118,286
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	10,618	8,355
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	6,492	11,035
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	23,328	10,965
Accruing Current Balances	2,619,507	2,924,522
Nonaccrual Loans	20,637	13,816
Total Loans	2,663,472	2,949,303	2,298,372
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	22,976	7,313
Commercial and industrial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	352	3,652
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,491	4,012
Accruing Current Balances	1,328,474	1,323,355
Nonaccrual Loans	4,024	5,360
Total Loans	1,333,989	1,332,727	1,313,193
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,491	3,120
Owner-occupied commercial | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	892
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	4,313	7,034
Accruing Current Balances	1,970,011	2,061,853
Nonaccrual Loans	1,642	17,175
Total Loans	1,975,966	2,086,062	2,223,117
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	4,229	5,944
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	84	1,090
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	371
Accruing Current Balances	784,101	715,904
Nonaccrual Loans	0	0
Total Loans	784,101	716,275	$ 626,253
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	371
Construction | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	0
Student loans | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 14,900	$ 18,200