DERIVATIVE FINANCIAL INSTRUMENTS - Interest rate swap positions (Details) - Swap [Member] - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Investment Securities Class [Domain]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 1,760,000,000	$ 1,260,000,000
Average Fixed Pay Rate	1.85%	1.70%
Average Receive Rate	2.66%	2.64%
Assets, at Fair Value	$ 12,035,000	$ 14,557,000
Average Maturity	2 years 6 months 9 days	1 year 9 months 17 days
1-3 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 1,100,000,000	$ 1,000,000,000
Average Fixed Pay Rate	1.67%	1.62%
Average Receive Rate	2.69%	2.63%
Assets, at Fair Value	$ 8,757,000	$ 10,365,000
Average Maturity	1 year 3 months 20 days	1 year 4 months 21 days
3-5 Years
Derivatives, Fair Value [Line Items]
Notional Amount	$ 660,000,000	$ 260,000,000
Average Fixed Pay Rate	2.15%	2.01%
Average Receive Rate	2.63%	2.68%
Assets, at Fair Value	$ 3,278,000	$ 4,192,000
Average Maturity	4 years 6 months 23 days	3 years 4 months 13 days

Nominal or face amount used to calculate payments on the derivative asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
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-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

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-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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