FAIR VALUE MEASUREMENTS - Unobservable Inputs Non-recurring (Details) - Non-recurring - Level 3 - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Quantitative information about the significant unobservable inputs within Level 3
Impaired loans	$ 22,766,000	$ 23,853,000
Capitalized servicing rights	12,400,000	12,527,000
Total	35,166,000	36,380,000
Impaired Loans \| Fair Value of Collateral
Quantitative information about the significant unobservable inputs within Level 3
Impaired loans	$ 22,766,000	$ 23,853,000
Impaired Loans \| Minimum \| Fair Value of Collateral
Quantitative information about the significant unobservable inputs within Level 3
Loss severity (as percent)	0.08%	0.21%
Appraised value	$ 10,600	$ 10,900
Impaired Loans \| Maximum \| Fair Value of Collateral
Quantitative information about the significant unobservable inputs within Level 3
Loss severity (as percent)	38.43%	38.72%
Appraised value	$ 5,946,000	$ 5,967,000
Impaired Loans \| Weighted Average \| Fair Value of Collateral
Quantitative information about the significant unobservable inputs within Level 3
Loss severity (as percent)	2.41%	3.40%
Appraised value	$ 2,143,000	$ 2,197,000
Capitalized Servicing Rights \| Discounted Cash Flow
Quantitative information about the significant unobservable inputs within Level 3
Capitalized servicing rights	$ 12,400,000	$ 12,527,000
Capitalized Servicing Rights \| Minimum \| Discounted Cash Flow
Quantitative information about the significant unobservable inputs within Level 3
Constant prepayment rate (CPR) (as percent)		7.78%
Discount Rate (as percent)		10.00%
Capitalized Servicing Rights \| Maximum \| Discounted Cash Flow
Quantitative information about the significant unobservable inputs within Level 3
Constant prepayment rate (CPR) (as percent)		12.78%
Discount Rate (as percent)		13.28%
Capitalized Servicing Rights \| Weighted Average \| Discounted Cash Flow
Quantitative information about the significant unobservable inputs within Level 3
Constant prepayment rate (CPR) (as percent)		10.38%
Discount Rate (as percent)		11.72%

Represents the appraised value used as an input to measure fair value.

No definition available.

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- Definition

Represents the amount of impaired loans assets reported on the balance sheet at period end measured at fair value by the entity.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

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.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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