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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) |  | 12 Months Ended
--- | --- | ---
Jul. 01, 2017 USD ($) bank | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Number of community banking office locations, ACNB |  | 22 |  |
Number of community banking office locations, NWSB |  | 7 |  |
Assets Held-in-trust |  | $ 327,000,000 | $ 195,000,000 |
Total Loans Receivable |  | $ 1,244,170,000 | 907,910,000 | $ 852,960,000
Number of minimum months for corporation to order updated valuation |  | 18 months |  |
Post-employment benefit cost for continuing life insurance |  | $ 96,000 | 96,000 | $ 46,000
Foreclosed assets held for resale |  | $ 436,000 | $ 256,000 |
Weighted average number of shares outstanding, basic | shares |  | 6,543,756 | 6,051,579 | 6,026,224
ASU 2018-02 adjustment |  | $ 0 |  |
Restricted Stock |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Cumulative amount of shares issued under the plan | shares |  | 19,301 |  |
Compensation expense |  | $ 133,000 | $ 59,000 |
Shares vested | shares |  | 13,235 |  |
Shares expected to vest over next two years | shares |  | 6,066 |  |
Weighted average remaining contractual term |  | 2 years |  |
Credit Concentration Risk |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Total Loans Receivable |  | $ 269,080,000 |  |
Concentration Risk, Percentage |  | 21.60% |  |
Minimum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Intangible asset amortization life |  | 5 years |  |
Maximum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Intangible asset amortization life |  | 15 years |  |
Residential Mortgage and Commercial Loans |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Threshold period past due to discontinue accrual interest on financing receivable |  | 90 days |  |
Consumer |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Total Loans Receivable |  | $ 16,337,000 | 14,704,000 | $ 14,588,000
Threshold period past due for charge off of financing receivable |  | 120 days |  |
Commercial real estate |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Total Loans Receivable |  | $ 492,759,000 | $ 318,980,000 | $ 289,899,000
Threshold period past due for Corporation to order updated valuation on financing receivable |  | 90 days |  |
Number of previous months with no updated valuation completed for Corporation to update validation |  | 12 months |  |
Number of maximum days of loan being classified as impaired for Corporation to order third party valuation |  | 30 days |  |
Commercial real estate | Maximum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Financing receivable term |  | 20 years |  |
Loan-to-value ratio (no greater than) |  | 80.00% |  |
Residential Mortgage |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Loan-to-value ratio that requires private mortgage insurance (in excess of) |  | 80.00% |  |
Foreclosed assets held for resale |  | $ 264,000 |  |
Residential Mortgage | Maximum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Financing receivable term |  | 30 years |  |
Loan-to-value ratio (no greater than) |  | 80.00% |  |
Home Equity Line of Credit | Maximum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Financing receivable term |  | 20 years |  |
Loan-to-value ratio (no greater than) |  | 90.00% |  |
Buildings |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Useful life |  | 40 years |  |
Building Remodels and Additions |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Useful life |  | 15 years |  |
Bank Equipment | Minimum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Useful life |  | 5 years |  |
Bank Equipment | Maximum |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Useful life |  | 15 years |  |
Retained Earnings |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
ASU 2018-02 adjustment |  | $ 1,183,000 |  |
New Windsor |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |
Number of community banking office locations, NWSB | bank | 7 |  |  |
Interest rate fair value discount | $ 731,000 |  |  |
Credit fair value discount | $ 4,501,000 |  |  |

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