DERIVATIVE FINANCIAL INSTRUMENTS - Interest rate swap positions (Details) - Swap [Member] - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Investment Securities Class [Domain]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 1,010,000,000	$ 1,010,000,000
Average Fixed Pay Rate	1.43%	1.43%
Average Receive Rate	1.89%	1.31%
Assets, at Fair Value	$ 25,107,000	$ 8,102,000
Average Maturity	3 years 1 month 2 days	2 years 10 months 2 days
1-3 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 650,000,000	$ 650,000,000
Average Fixed Pay Rate	1.09%	1.09%
Average Receive Rate	1.81%	1.31%
Assets, at Fair Value	$ 17,232,000	$ 10,318,000
Average Maturity	1 year 9 months 20 days	2 years 20 days
3-5 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 360,000,000	$ 360,000,000
Average Fixed Pay Rate	2.05%	2.05%
Average Receive Rate	2.03%	1.32%
Assets, at Fair Value	$ 7,875,000
Liabilities, at Fair Value		$ (2,216,000)
Average Maturity	4 years 3 days	4 years 3 months 3 days