DERIVATIVE FINANCIAL INSTRUMENTS - Interest rate swap positions (Details) - Swap [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investment Securities Class [Domain]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 1,010,000,000
Average Fixed Pay Rate	1.43%
Average Receive Rate	1.45%
Assets, at Fair Value	$ 13,530,000
Average Maturity	2 years 10 months 2 days
1-3 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 650,000,000
Average Fixed Pay Rate	1.09%
Average Receive Rate	1.41%
Assets, at Fair Value	$ 11,828,000
Average Maturity	2 years 20 days
3-5 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 360,000,000	$ 700,000,000
Average Fixed Pay Rate	2.05%	1.20%
Average Receive Rate	1.53%	0.91%
Assets, at Fair Value	$ 1,702,000	$ 9,500,000
Average Maturity	4 years 3 months 3 days	3 years 4 months 10 days