ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Residential and Consumer Credit Exposure (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 8,464,562
Total		100.00%
Accrued troubled debt restructured loans		$ 14,000
Reverse mortgage, fair value	$ 12,500	16,600	$ 17,700
Acquired nonimpaired loans		3,200,000	$ 3,421,089
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 2,128,610
Total		100.00%
Acquired nonimpaired loans		$ 1,100,000
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		999,879
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 1,128,731
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total		0.00%
Acquired impaired | Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 9,453
Total		0.00%
Acquired impaired | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 7,326
Acquired impaired | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		2,127
Nonperforming | Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 20,232
Total		1.00%
Nonperforming | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 12,858
Nonperforming | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		7,374
Performing | Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 2,098,925
Total		99.00%
Performing | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 979,695
Performing | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans		$ 1,119,230