DERIVATIVE FINANCIAL INSTRUMENTS - Interest rate swap positions (Details) - Swap [Member] - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 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	1.31%
Assets, at Fair Value	$ 8,102,000
Average Maturity	3 years 1 month 2 days
1-3 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 650,000,000
Average Fixed Pay Rate	1.09%
Average Receive Rate	1.31%
Assets, at Fair Value	$ 10,318,000
Average Maturity	2 years 3 months 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	1.32%	0.91%
Assets, at Fair Value		$ 9,500,000
Liabilities, at Fair Value	$ (2,216,000)
Average Maturity	4 years 6 months 3 days	3 years 4 months 10 days