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LOANS - Summary of Past Due Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total loans	$ 9,941,828	$ 9,043,253
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total loans	4,005,347	3,400,221
Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Total loans	359,493	353,012
Commercial real estate | Other
Financing Receivable, Past Due [Line Items]
Total loans	3,645,854	3,047,209
Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Total loans	1,987,297	1,980,046
Residential mortgages
Financing Receivable, Past Due [Line Items]
Total loans	2,882,380	2,566,424
Residential mortgages | Construction
Financing Receivable, Past Due [Line Items]
Total loans	17,328	9,756
Residential mortgages | 1-4 family
Financing Receivable, Past Due [Line Items]
Total loans	2,865,052	2,556,668
Consumer
Financing Receivable, Past Due [Line Items]
Total loans	1,066,804	1,096,562
Consumer | Home equity
Financing Receivable, Past Due [Line Items]
Total loans	403,830	376,680
Consumer | Auto and other
Financing Receivable, Past Due [Line Items]
Total loans	662,974	719,882
Business Activities Loans
Financing Receivable, Past Due [Line Items]
Total Past Due	43,614	42,440
Current	7,250,791	7,324,304
Total loans	7,294,405	7,366,744
Past Due 90 days and Accruing	2,845	1,063
Business Activities Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	7,015	11,399
Business Activities Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,050	3,500
Business Activities Loans | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	33,549	27,541
Business Activities Loans | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	18,241	20,022
Current	2,641,257	2,568,689
Total loans	2,659,498	2,588,711
Past Due 90 days and Accruing	0	993
Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	300,996	327,792
Total loans	300,996	327,792
Past Due 90 days and Accruing	0	0
Business Activities Loans | Commercial real estate | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	18,241	20,022
Current	2,340,261	2,240,897
Total loans	2,358,502	2,260,919
Past Due 90 days and Accruing	0	993
Business Activities Loans | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	388	913
Business Activities Loans | Commercial real estate | 30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Commercial real estate | 30-59 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	388	913
Business Activities Loans | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	478	276
Business Activities Loans | Commercial real estate | 60-89 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Commercial real estate | 60-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	478	276
Business Activities Loans | Commercial real estate | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	17,375	18,833
Business Activities Loans | Commercial real estate | 90 Days or Greater Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Commercial real estate | 90 Days or Greater Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	17,375	18,833
Business Activities Loans | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Total Past Due	12,744	10,305
Current	1,459,022	1,503,233
Total loans	1,471,766	1,513,538
Past Due 90 days and Accruing	1,422	4
Business Activities Loans | Commercial and industrial loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,071	4,694
Business Activities Loans | Commercial and industrial loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	784	975
Business Activities Loans | Commercial and industrial loans | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	9,889	4,636
Business Activities Loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	4,914	4,690
Current	2,272,701	2,322,608
Total loans	2,277,615	2,327,298
Past Due 90 days and Accruing	1,258	66
Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	9,170	9,582
Total loans	9,170	9,582
Past Due 90 days and Accruing	0	0
Business Activities Loans | Residential mortgages | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	4,914	4,690
Current	2,263,531	2,313,026
Total loans	2,268,445	2,317,716
Past Due 90 days and Accruing	1,258	66
Business Activities Loans | Residential mortgages | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	867	1,631
Business Activities Loans | Residential mortgages | 30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Residential mortgages | 30-59 Days Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	867	1,631
Business Activities Loans | Residential mortgages | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	543	1,619
Business Activities Loans | Residential mortgages | 60-89 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Residential mortgages | 60-89 Days Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	543	1,619
Business Activities Loans | Residential mortgages | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,504	1,440
Business Activities Loans | Residential mortgages | 90 Days or Greater Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Business Activities Loans | Residential mortgages | 90 Days or Greater Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	3,504	1,440
Business Activities Loans | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	7,715	7,423
Current	877,811	929,774
Total loans	885,526	937,197
Past Due 90 days and Accruing	165	0
Business Activities Loans | Consumer | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	1,752	1,566
Current	279,502	288,395
Total loans	281,254	289,961
Past Due 90 days and Accruing	165	0
Business Activities Loans | Consumer | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	5,963	5,857
Current	598,309	641,379
Total loans	604,272	647,236
Past Due 90 days and Accruing	0	0
Business Activities Loans | Consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,689	4,161
Business Activities Loans | Consumer | 30-59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	191	618
Business Activities Loans | Consumer | 30-59 Days Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	3,498	3,543
Business Activities Loans | Consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,245	630
Business Activities Loans | Consumer | 60-89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	582	15
Business Activities Loans | Consumer | 60-89 Days Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	663	615
Business Activities Loans | Consumer | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,781	2,632
Business Activities Loans | Consumer | 90 Days or Greater Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	979	933
Business Activities Loans | Consumer | 90 Days or Greater Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	1,802	1,699
Acquired Loans
Financing Receivable, Past Due [Line Items]
Total Past Due	12,735	14,227
Acquired Credit Impaired	74,092	47,288
Total loans	2,647,423	1,676,509
Past Due 90 days and Accruing	2,812	1,967
Acquired Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,570	4,685
Acquired Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,286	1,628
Acquired Loans | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	7,879	7,914
Acquired Loans | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	6,125	7,913
Acquired Credit Impaired	29,623	11,994
Total loans	1,345,849	811,510
Past Due 90 days and Accruing	2,441	1,652
Acquired Loans | Commercial real estate | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Credit Impaired	1,411	0
Total loans	58,497	25,220
Past Due 90 days and Accruing	0	0
Acquired Loans | Commercial real estate | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	6,125	7,913
Acquired Credit Impaired	28,212	11,994
Total loans	1,287,352	786,290
Past Due 90 days and Accruing	2,441	1,652
Acquired Loans | Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	998	2,603
Acquired Loans | Commercial real estate | 30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Commercial real estate | 30-59 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	998	2,603
Acquired Loans | Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	695	1,127
Acquired Loans | Commercial real estate | 60-89 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Commercial real estate | 60-89 Days Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	695	1,127
Acquired Loans | Commercial real estate | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,432	4,183
Acquired Loans | Commercial real estate | 90 Days or Greater Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Commercial real estate | 90 Days or Greater Past Due | Other
Financing Receivable, Past Due [Line Items]
Total Past Due	4,432	4,183
Acquired Loans | Commercial and industrial loans
Financing Receivable, Past Due [Line Items]
Total Past Due	2,120	1,879
Acquired Credit Impaired	31,456	29,539
Total loans	515,531	466,508
Past Due 90 days and Accruing	214	144
Acquired Loans | Commercial and industrial loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,055	217
Acquired Loans | Commercial and industrial loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	62	147
Acquired Loans | Commercial and industrial loans | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,003	1,515
Acquired Loans | Residential mortgages
Financing Receivable, Past Due [Line Items]
Total Past Due	2,866	2,444
Acquired Credit Impaired	12,050	4,888
Total loans	604,765	239,126
Past Due 90 days and Accruing	75	75
Acquired Loans | Residential mortgages | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Credit Impaired	0	0
Total loans	8,158	174
Past Due 90 days and Accruing	0	0
Acquired Loans | Residential mortgages | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	2,866	2,444
Acquired Credit Impaired	12,050	4,888
Total loans	596,607	238,952
Past Due 90 days and Accruing	75	75
Acquired Loans | Residential mortgages | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	970	1,382
Acquired Loans | Residential mortgages | 30-59 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Residential mortgages | 30-59 Days Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	970	1,382
Acquired Loans | Residential mortgages | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	488	144
Acquired Loans | Residential mortgages | 60-89 Days Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Residential mortgages | 60-89 Days Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	488	144
Acquired Loans | Residential mortgages | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,408	918
Acquired Loans | Residential mortgages | 90 Days or Greater Past Due | Construction
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Acquired Loans | Residential mortgages | 90 Days or Greater Past Due | 1-4 family
Financing Receivable, Past Due [Line Items]
Total Past Due	1,408	918
Acquired Loans | Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	1,624	1,991
Acquired Credit Impaired	963	867
Total loans	181,278	159,365
Past Due 90 days and Accruing	82	96
Acquired Loans | Consumer | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	1,106	1,189
Acquired Credit Impaired	700	553
Total loans	122,576	86,719
Past Due 90 days and Accruing	82	0
Acquired Loans | Consumer | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	518	802
Acquired Credit Impaired	263	314
Total loans	58,702	72,646
Past Due 90 days and Accruing	0	96
Acquired Loans | Consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	547	483
Acquired Loans | Consumer | 30-59 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	379	290
Acquired Loans | Consumer | 30-59 Days Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	168	193
Acquired Loans | Consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	41	210
Acquired Loans | Consumer | 60-89 Days Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	0	148
Acquired Loans | Consumer | 60-89 Days Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	41	62
Acquired Loans | Consumer | 90 Days or Greater Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,036	1,298
Acquired Loans | Consumer | 90 Days or Greater Past Due | Home equity
Financing Receivable, Past Due [Line Items]
Total Past Due	727	751
Acquired Loans | Consumer | 90 Days or Greater Past Due | Auto and other
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 309	$ 547

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