ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 63,317	$ 48,833
Accruing Current Balances	9,233,407	8,365,872
Nonaccrual Loans	25,835
Total Loans	9,322,559		$ 8,518,915
Acquired Impaired Loans		26,935
Nonaccrual Loans	$ 15	22,922
Total Loans		$ 8,464,562
Percent past due	0.68%	0.58%
Accruing Current Balances, % of Total Loans	99.04%	98.83%
Percent of Nonaccrual Loans With No Allowance for Credit Losses	0.28%
Acquired Impaired Loans, % of Total Loans		0.32%
Nonaccrual Loans, % of Total Loans		0.27%
% of Total Loans	100.00%
% of Total Loans		100.00%
Reverse mortgage, fair value	$ 12,500	$ 16,600	17,700
Acquired nonimpaired loans		3,200,000	3,421,089
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 51,431	$ 32,683
Percent past due	0.55%	0.39%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 11,886	$ 16,150
Percent past due	0.13%	0.19%
Commercial
Financing Receivable, Past Due [Line Items]
Acquired nonimpaired loans		$ 2,200,000
Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 2,673	7,170
Accruing Current Balances	839,240	980,893
Nonaccrual Loans	3,082
Total Loans	844,995		1,033,896
Acquired Impaired Loans		7,326
Nonaccrual Loans		4,490
Total Loans		999,879
Acquired nonimpaired loans			876,089
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	2,673	6,733
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	437
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	16,558	25,909
Accruing Current Balances	1,149,944	1,098,980
Nonaccrual Loans	2,389
Total Loans	1,168,891		1,138,873
Acquired Impaired Loans		2,127
Nonaccrual Loans		1,715
Total Loans		1,128,731
Acquired nonimpaired loans			241,633
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	10,282	13,164
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	6,276	12,745
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	18,354	8,327
Accruing Current Balances	3,097,310	2,214,506
Nonaccrual Loans	14,690
Total Loans	3,130,354		2,286,351
Acquired Impaired Loans		1,564
Nonaccrual Loans		11,031
Total Loans		2,235,428
Acquired nonimpaired loans			605,804
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	17,314	6,289
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	1,040	2,038
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	6,244	2,329
Accruing Current Balances	1,334,228	1,283,320
Nonaccrual Loans	4,022
Total Loans	1,344,494		1,273,408
Acquired Impaired Loans		6,757
Nonaccrual Loans		4,060
Total Loans		1,296,466
Acquired nonimpaired loans			322,030
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	3,073	1,498
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	3,171	831
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	16,689	5,098
Accruing Current Balances	2,149,167	2,207,582
Nonaccrual Loans	1,652
Total Loans	2,167,508		2,271,985
Acquired Impaired Loans		8,670
Nonaccrual Loans		1,626
Total Loans		2,222,976
Acquired nonimpaired loans			1,237,282
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	15,290	4,999
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,399	99
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	2,799	0
Accruing Current Balances	663,518	580,591
Nonaccrual Loans	0
Total Loans	666,317		514,402
Acquired Impaired Loans		491
Nonaccrual Loans		0
Total Loans		581,082
Acquired nonimpaired loans			$ 138,251
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	2,799	0
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	$ 15,000	$ 22,300