ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 36,248	$ 48,833
Accruing Current Balances	9,273,946	8,365,872
Nonaccrual Loans	26,175
Total Loans	9,336,369		$ 8,601,678
Acquired Impaired Loans		26,935
Nonaccrual Loans	$ 16	22,922
Total Loans		$ 8,464,562
Percent past due	0.39%	0.58%
Accruing Current Balances, % of Total Loans	99.33%	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	$ 16,100	$ 16,600	15,900
Acquired nonimpaired loans		3,200,000	3,699,730
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 27,647	$ 32,683
Percent past due	0.30%	0.39%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 8,601	$ 16,150
Percent past due	0.09%	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	$ 1,679	7,170
Accruing Current Balances	890,063	980,893
Nonaccrual Loans	3,118
Total Loans	894,860		1,065,784
Acquired Impaired Loans		7,326
Nonaccrual Loans		4,490
Total Loans		999,879
Acquired nonimpaired loans			912,288
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,504	6,733
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	175	437
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	11,626	25,909
Accruing Current Balances	1,119,429	1,098,980
Nonaccrual Loans	2,316
Total Loans	1,133,371		1,126,733
Acquired Impaired Loans		2,127
Nonaccrual Loans		1,715
Total Loans		1,128,731
Acquired nonimpaired loans			267,955
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	5,473	13,164
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	6,153	12,745
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	8,657	8,327
Accruing Current Balances	3,142,796	2,214,506
Nonaccrual Loans	15,381
Total Loans	3,166,834		2,340,524
Acquired Impaired Loans		1,564
Nonaccrual Loans		11,031
Total Loans		2,235,428
Acquired nonimpaired loans			738,579
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	7,577	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	1,080	2,038
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	5,395	2,329
Accruing Current Balances	1,327,646	1,283,320
Nonaccrual Loans	4,212
Total Loans	1,337,253		1,280,894
Acquired Impaired Loans		6,757
Nonaccrual Loans		4,060
Total Loans		1,296,466
Acquired nonimpaired loans			99,326
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	5,198	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	197	831
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	8,891	5,098
Accruing Current Balances	2,155,508	2,207,582
Nonaccrual Loans	1,148
Total Loans	2,165,547		2,246,047
Acquired Impaired Loans		8,670
Nonaccrual Loans		1,626
Total Loans		2,222,976
Acquired nonimpaired loans			1,464,739
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	7,895	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	996	99
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	0
Accruing Current Balances	638,504	580,591
Nonaccrual Loans	0
Total Loans	638,504		541,696
Acquired Impaired Loans		491
Nonaccrual Loans		0
Total Loans		581,082
Acquired nonimpaired loans			$ 216,843
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0	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	$ 10,500	$ 22,300