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