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	Mar. 31, 2018	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 57,410		$ 13,343
Accruing Current Balances	8,561,079		4,835,916
Acquired Impaired Loans	42,490	$ 20,154	14,944
Nonaccrual Loans	32,038		30,054
Total Loans	$ 8,693,017	4,834,427	$ 4,894,257
30-59 Days Past Due and Still Accruing, % of Total Loans	0.40%		0.17%
60-89 Days Past Due and Still Accruing, % of Total Loans	0.11%		0.08%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.15%		0.02%
Total Past Due And Still Accruing, % of Total Loans	0.66%		0.27%
Accruing Current Balances, % of Total Loans	98.48%		98.81%
Acquired Impaired Loans, % of Total Loans	0.49%		0.31%
Nonaccrual Loans, % of Total Loans	0.37%		0.61%
% of Total Loans	100.00%		100.00%
Reverse mortgage, fair value	$ 16,200	20,000	$ 16,500
Acquired nonimpaired loans	3,933,340	534,841	430,000
Commercial and Industrial (1)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	3,774		4,717
Accruing Current Balances	2,194,854		1,452,185
Acquired Impaired Loans	6,130	3,870	1,531
Nonaccrual Loans	15,330		14,056
Total Loans	2,220,088	1,489,563	1,472,489
Acquired nonimpaired loans	782,160	107,183
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	13,894		2,975
Accruing Current Balances	1,074,517		194,960
Acquired Impaired Loans	8,483	777	761
Nonaccrual Loans	2,482		2,854
Total Loans	1,099,376	227,113	201,550
Acquired nonimpaired loans	949,804	67,722
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	28,724		1,600
Accruing Current Balances	1,097,386		677,182
Acquired Impaired Loans	3,456	249	151
Nonaccrual Loans	2,193		2,006
Total Loans	1,131,759	582,722	680,939
Acquired nonimpaired loans	295,450	33,152
30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	34,410		8,426
30–59 Days Past Due and Still Accruing | Commercial and Industrial (1)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	2,941		3,653
30–59 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	11,404		1,970
30–59 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	10,470		525
60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	10,024		4,082
60–89 Days Past Due and Still Accruing | Commercial and Industrial (1)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	833		993
60–89 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,751		345
60–89 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	6,017		971
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	12,976		835
Greater Than 90 Days Past Due and Still Accruing | Commercial and Industrial (1)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		71
Greater Than 90 Days Past Due and Still Accruing | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	739		660
Greater Than 90 Days Past Due and Still Accruing | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	12,237		104
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	3,109		1,598
Accruing Current Balances	1,300,383		1,049,722
Acquired Impaired Loans	4,070	5,147	4,248
Nonaccrual Loans	5,383		4,406
Total Loans	1,312,945	1,080,662	1,059,974
Acquired nonimpaired loans	105,154	133,007
Owner-occupied commercial | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	3,109		733
Owner-occupied commercial | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		865
Owner-occupied commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Commercial mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	5,962		2,296
Accruing Current Balances	2,323,606		1,148,988
Acquired Impaired Loans	19,715	9,210	7,504
Nonaccrual Loans	3,869		3,951
Total Loans	2,353,152	1,164,424	1,162,739
Acquired nonimpaired loans	1,575,527	178,518
Commercial mortgages | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	5,848		1,388
Commercial mortgages | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	114		908
Commercial mortgages | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	0		0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,947		157
Accruing Current Balances	570,333		312,879
Acquired Impaired Loans	636	901	749
Nonaccrual Loans	2,781		2,781
Total Loans	575,697	289,943	316,566
Acquired nonimpaired loans	225,245	$ 15,259
Construction | 30–59 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	638		157
Construction | 60–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	1,309		0
Construction | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due And Still Accruing	$ 0		$ 0