LOANS - Narrative (Details) - USD ($)		9 Months Ended		12 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income on nonaccrual loans		$ 0
Nonaccrual loans		$ 138,163,000		$ 75,124,000
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Loans modified that are not troubled debt restructurings		$ 313,600,000	$ 168,600,000
COVID-19 Deferrals		648,516,000
Loans Excluding Purchased Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring		132,900,000		$ 35,200,000
Troubled debt restructurings, previous charge-offs, excluding purchased loans		1,200,000		1,900,000
Allowance for loan losses allocated to troubled debt restructuring		9,400,000		3,700,000
Troubled debt restructurings principal balances		103,409,000	6,696,000
Financing receivable, modifications, subsequent default, recorded investment		2,800,000	2,100,000
Minimum | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans		100,000	100,000	100,000
Troubled debt restructuring loans		100,000	$ 100,000	$ 100,000
Real Estate, Equipment and Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral-dependent Loans		$ 164,000,000.0