ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	$ 52,946		$ 13,343
Accruing Current Balances	8,395,736		4,835,916
Acquired Impaired Loans	31,815	$ 17,493	14,944
Nonaccrual Loans	38,418		30,054
Loans and Leases Receivable, Gross	$ 8,518,915	4,919,277	$ 4,894,257
30-59 Days Past Due and Still Accruing, % of Total Loans	0.38%		0.17%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.08%		0.08%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.16%		0.02%
Total Past Due And Still Accruing, % of Total Loans	0.62%		0.27%
Accruing Current Balances, % of Total Loans	98.55%		98.81%
Acquired Impaired Loans, % of Total Loans	0.37%		0.31%
Nonaccrual Loans, % of Total Loans	0.45%		0.61%
% of Total Loans	100.00%		100.00%
Reverse mortgage, fair value	$ 17,700	16,600	$ 16,500
Acquired nonimpaired loans	3,421,089	465,438	430,000
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	8,833		4,717
Accruing Current Balances	2,254,847		1,452,185
Acquired Impaired Loans	1,664	2,070	1,531
Nonaccrual Loans	21,007		14,056
Loans and Leases Receivable, Gross	2,286,351	1,506,675	1,472,489
Acquired nonimpaired loans	605,804	97,552
Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	8,481		2,975
Accruing Current Balances	1,013,608		194,960
Acquired Impaired Loans	7,655	766	761
Nonaccrual Loans	4,152		2,854
Loans and Leases Receivable, Gross	1,033,896	206,726	201,550
Acquired nonimpaired loans	876,089	60,407
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	23,200		1,600
Accruing Current Balances	1,111,329		677,182
Acquired Impaired Loans	2,545	153	151
Nonaccrual Loans	1,799		2,006
Loans and Leases Receivable, Gross	1,138,873	653,704	680,939
Acquired nonimpaired loans	241,633	24,568
30–59 Days Past Due  and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	32,395		8,426
30–59 Days Past Due  and Still  Accruing | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	7,665		3,653
30–59 Days Past Due  and Still  Accruing | Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	7,475		1,970
30–59 Days Past Due  and Still  Accruing | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	8,152		525
60–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	6,842		4,082
60–89 Days Past Due and Still  Accruing | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	640		993
60–89 Days Past Due and Still  Accruing | Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	758		345
60–89 Days Past Due and Still  Accruing | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	3,104		971
Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	13,709		835
Greater  Than 90 Days Past Due and Still Accruing | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	528		71
Greater  Than 90 Days Past Due and Still Accruing | Residential
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	248		660
Greater  Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	11,944		104
Owner-occupied commercial
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	3,573		1,598
Accruing Current Balances	1,253,763		1,049,722
Acquired Impaired Loans	6,809	4,816	4,248
Nonaccrual Loans	9,263		4,406
Loans and Leases Receivable, Gross	1,273,408	1,085,404	1,059,974
Acquired nonimpaired loans	322,030	119,403
Owner-occupied commercial | 30–59 Days Past Due  and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	2,183		733
Owner-occupied commercial | 60–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,390		865
Owner-occupied commercial | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	7,839		2,296
Accruing Current Balances	2,249,294		1,148,988
Acquired Impaired Loans	12,655	8,951	7,504
Nonaccrual Loans	2,197		3,951
Loans and Leases Receivable, Gross	2,271,985	1,133,281	1,162,739
Acquired nonimpaired loans	1,237,282	156,483
Commercial mortgages | 30–59 Days Past Due  and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	5,900		1,388
Commercial mortgages | 60–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	950		908
Commercial mortgages | Greater  Than 90 Days Past Due and Still Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	989		0
Construction
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,020		157
Accruing Current Balances	512,895		312,879
Acquired Impaired Loans	487	737	749
Nonaccrual Loans	0		2,781
Loans and Leases Receivable, Gross	514,402	333,487	316,566
Acquired nonimpaired loans	138,251	$ 7,025
Construction | 30–59 Days Past Due  and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	1,020		157
Construction | 60–89 Days Past Due and Still  Accruing
Financing Receivable, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction | 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	$ 18,800