ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Mar. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		$ 48,833	$ 52,220
Accruing Current Balances		8,365,872	8,463,516
Nonaccrual Loans(1)			19,250
Acquired Impaired Loans	$ 42,490	26,935
Nonaccrual Loans		22,922	18
Total Loans			8,534,986
Total Loans	8,693,017	$ 8,464,562	$ 8,534,986
Percent past due		0.58%	0.61%
Percent of Nonaccrual Loans With No Allowance for Credit Losses			0.23%
Accruing Current Balances, % of Total Loans		98.83%	99.16%
Acquired Impaired Loans, % of Total Loans		0.32%
Nonaccrual Loans, % of Total Loans		0.27%
% of Total Loans		100.00%	100.00%
Reverse mortgage, fair value	16,200	$ 16,600	$ 15,800
Acquired nonimpaired loans	3,933,340	3,200,000
30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		$ 32,683	$ 37,871
Percent past due		0.39%	0.44%
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		$ 16,150	$ 14,349
Percent past due		0.19%	0.17%
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		7,170	$ 4,207
Accruing Current Balances		980,893	947,827
Nonaccrual Loans(1)			3,731
Acquired Impaired Loans	8,483	7,326
Nonaccrual Loans		4,490
Total Loans			955,765
Total Loans	1,099,376	999,879	955,765
Acquired nonimpaired loans	949,804
Residential | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		6,733	4,089
Residential | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		437	118
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		25,909	24,236
Accruing Current Balances		1,098,980	1,091,952
Nonaccrual Loans(1)			2,098
Acquired Impaired Loans	3,456	2,127
Nonaccrual Loans		1,715
Total Loans			1,118,286
Total Loans	1,131,759	1,128,731	1,118,286
Acquired nonimpaired loans	295,450
Consumer | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		13,164	13,077
Consumer | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		12,745	11,159
Commercial and industrial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		8,327	8,609
Accruing Current Balances		2,214,506	2,281,204
Nonaccrual Loans(1)			8,559
Acquired Impaired Loans	6,130	1,564
Nonaccrual Loans		11,031
Total Loans			2,298,372
Total Loans	2,220,088	2,235,428	2,298,372
Acquired nonimpaired loans	782,160
Commercial and industrial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		6,289	8,526
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		2,038	83
Owner-occupied commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		2,329	6,250
Accruing Current Balances		1,283,320	1,303,392
Nonaccrual Loans(1)			3,551
Acquired Impaired Loans	4,070	6,757
Nonaccrual Loans		4,060
Total Loans			1,313,193
Total Loans	1,312,945	1,296,466	1,313,193
Acquired nonimpaired loans	105,154
Owner-occupied commercial | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		1,498	6,009
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		831	241
Commercial mortgages | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		5,098	6,520
Accruing Current Balances		2,207,582	2,215,286
Nonaccrual Loans(1)			1,311
Acquired Impaired Loans	19,715	8,670
Nonaccrual Loans		1,626
Total Loans			2,223,117
Total Loans	2,353,152	2,222,976	2,223,117
Acquired nonimpaired loans	1,575,527
Commercial mortgages | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		4,999	5,840
Commercial mortgages | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		99	680
Construction | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		0	2,398
Accruing Current Balances		580,591	623,855
Nonaccrual Loans(1)			0
Acquired Impaired Loans	636	491
Nonaccrual Loans		0
Total Loans			626,253
Total Loans	575,697	581,082	626,253
Acquired nonimpaired loans	$ 225,245
Construction | Commercial | 30–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		0	330
Construction | Commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		0	2,068
Student loans | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing		$ 22,300	$ 21,500