LOANS AND ALLOWANCE FOR CREDIT LOSSES Summary of Total Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	$ 8,982,336		$ 9,502,428	$ 9,502,428
Less: Allowance for credit losses on loans	134,414	$ 139,394	63,575	63,575
Net loans	8,847,922		9,438,853	9,438,853
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				5,874,777
Commercial | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	490,111		448,452	448,452
Less: Allowance for credit losses on loans	6,657	7,779		2,713
Commercial | Commercial multifamily
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	535,936		631,740	631,740
Less: Allowance for credit losses on loans	3,781	4,299		4,413
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				4,034,269
Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				429,806
Commercial real estate | Commercial real estate owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	595,791		673,308	673,308
Less: Allowance for credit losses on loans	10,995	11,552		4,880
Commercial real estate | Commercial real estate non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	2,258,935		2,189,780	2,189,780
Less: Allowance for credit losses on loans	32,774	34,707		16,344
Commercial real estate | Other commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				3,604,463
Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	1,819,175		1,522,059	1,840,508
Less: Allowance for credit losses on loans	16,928	17,779		17,243
Commercial and industrial loans | Commercial and industrial - other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	311,505		321,624
Less: Allowance for credit losses on loans	3,920	5,317		2,856
Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,685,472
Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				8,119
Residential mortgages | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	2,270,458		2,853,385	2,853,385
Less: Allowance for credit losses on loans	39,891	39,004		9,970
Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,677,353
Consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				942,179
Consumer loans | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	349,274		378,793	380,591
Less: Allowance for credit losses on loans	9,321	8,021		1,470
Consumer loans | Consumer other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	351,151		$ 483,287	483,287
Less: Allowance for credit losses on loans	10,147	$ 10,936		3,686
Business Activities Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				7,166,497
Business Activities Loans | Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				4,239,573
Business Activities Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,796,956
Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				382,014
Business Activities Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,414,942
Business Activities Loans | Commercial real estate | Other commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,414,942
Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				1,442,617
Business Activities Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,148,458
Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				4,641
Business Activities Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,143,817
Business Activities Loans | Consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				778,466
Business Activities Loans | Consumer loans | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				273,867
Acquired Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				2,335,931
Acquired Loans | Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				1,635,204
Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				1,237,313
Acquired Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				47,792
Acquired Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				1,189,521
Acquired Loans | Commercial real estate | Other commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				1,189,521
Acquired Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				397,891
Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				537,014
Acquired Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				3,478
Acquired Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				533,536
Acquired Loans | Consumer loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				163,713
Acquired Loans | Consumer loans | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans				$ 106,724
Small Business Administration (SBA), CARES Act, Paycheck Protection Program
Financing Receivable, Allowance for Credit Loss [Line Items]
Net loans	$ 708,100