LOANS (Narrative) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable		$ 42,400,000
Individually assessed loans, nonaccrual		500,000
Troubled debt restructured loans		$ 6,200,000	$ 7,600,000
Number of loans classified as doubtful | loan		0	0
Number of loans classified as loss | loan		0	0
Loans receivable held to maturity		$ 10,023,051,000	$ 8,367,917,000
Increase in allowance		131,606,000	$ 70,395,000	$ 61,963,000	$ 55,686,000
Loans secured by real estate property in process of foreclosure		$ 1,700,000
Loans delinquent 30-89 days as a percentage of total loans		0.23%	0.33%
Interest income on nonaccrual loans		$ 0
Nonaccrual loans with no related allowance		32,500,000
Threshold period past due, nonperforming status of loans and leases	90 days
Nonpass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable held to maturity			$ 556,769,000
Percent of nonaccrual loans			14.00%
Special Mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of loans			60.00%
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable held to maturity		477,754,000
Percent of loans			40.00%
Nonperforming Financial Instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructured loans		3,800,000	$ 3,800,000
Performing Financial Instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructured loans		2,400,000	$ 3,800,000
Paycheck Protection Program ("PPP") | Nonpass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable held to maturity		77,100,000
Increase in allowance		$ 0