DERIVATIVE FINANCIAL INSTRUMENTS - Interest rate swap positions (Details) - Swap [Member] - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Notional Amount	$ 1,355,000,000	$ 820,000,000
Average Fixed Pay Rate	0.79%	1.14%
Average Receive Rate	0.13%	0.23%
Net Estimated Fair Value	$ 14,747,000	$ (24,704,000)
Average Maturity	5 years 2 months 12 days	4 years 3 months 18 days
3-5 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 955,000,000	$ 620,000,000
Average Fixed Pay Rate	0.64%	1.29%
Average Receive Rate	0.13%	0.22%
Net Estimated Fair Value	$ 11,566,000	$ (23,760,000)
Average Maturity	4 years 3 months 18 days	3 years 7 months 6 days
More Than Five Years [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 400,000,000	$ 200,000,000
Average Fixed Pay Rate	1.16%	0.67%
Average Receive Rate	0.12%	0.23%
Net Estimated Fair Value	$ 3,181,000	$ (944,000)
Average Maturity	7 years 6 months	6 years 4 months 24 days