SECURITIES (Schedule of Unrealized Loss on Investments) (Details) $ in Thousands	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Securities Available for sale, Less than 12 Months: Fair Value	$ 51,045	$ 86,388
Securities Available for sale, Less than 12 Months: Unrealized Losses	509	1,713
Securities Available for sale, 12 Months or More: Fair Value	64,109	0
Securities Available for sale, 12 Months or More: Unrealized Losses	1,461	0
Securities Available for sale, Total: Fair Value	115,154	86,388
Securities Available for sale, Total Unrealized Loss	1,970	1,713
Securities Held to maturity, Less than 12 Months: Fair Value	9,931	25,902
Securities Held to maturity, Less than 12 Months: Unrealized Losses	44	379
Securities Held to maturity, 12 Months or More, Fair Value	21,986	0
Securities Held to maturity, 12 Months or More, Unrealized Losses	293	0
Securities Held to maturity, Total: Fair Value	31,917	25,902
Securities Held to maturity, Total Unrealized Losses	337	379
U.S. Government and agencies
Schedule of Investments [Line Items]
Securities Available for sale, Less than 12 Months: Fair Value	42,775	71,454
Securities Available for sale, Less than 12 Months: Unrealized Losses	445	1,529
Securities Available for sale, 12 Months or More: Fair Value	58,279	0
Securities Available for sale, 12 Months or More: Unrealized Losses	1,373	0
Securities Available for sale, Total: Fair Value	101,054	71,454
Securities Available for sale, Total Unrealized Loss	1,818	1,529
Securities Held to maturity, Less than 12 Months: Fair Value	4,985	12,946
Securities Held to maturity, Less than 12 Months: Unrealized Losses	15	54
Securities Held to maturity, 12 Months or More, Fair Value	10,916	0
Securities Held to maturity, 12 Months or More, Unrealized Losses	84	0
Securities Held to maturity, Total: Fair Value	15,901	12,946
Securities Held to maturity, Total Unrealized Losses	$ 99	54
Available for sale securities in unrealized loss positions \| Security	64
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	5.00%
Available for sale securities in unrealized loss positions for 12 months or more \| Security	32
Held to maturity securities in unrealized loss positions \| Security	9
Held to maturity securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	2.00%
Held to maturity securities in unrealized loss positions for 12 months or more \| Security	6
Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Securities Available for sale, Less than 12 Months: Fair Value	$ 7,228	8,966
Securities Available for sale, Less than 12 Months: Unrealized Losses	56	81
Securities Available for sale, 12 Months or More: Fair Value	2,845	0
Securities Available for sale, 12 Months or More: Unrealized Losses	45	0
Securities Available for sale, Total: Fair Value	10,073	8,966
Securities Available for sale, Total Unrealized Loss	101	81
Securities Held to maturity, Less than 12 Months: Fair Value	4,946	12,956
Securities Held to maturity, Less than 12 Months: Unrealized Losses	29	325
Securities Held to maturity, 12 Months or More, Fair Value	11,070	0
Securities Held to maturity, 12 Months or More, Unrealized Losses	209	0
Securities Held to maturity, Total: Fair Value	16,016	12,956
Securities Held to maturity, Total Unrealized Losses	$ 238	325
Available for sale securities in unrealized loss positions \| Security	15
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	2.00%
Available for sale securities in unrealized loss positions for 12 months or more \| Security	3
Held to maturity securities in unrealized loss positions \| Security	21
Held to maturity securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	3.00%
Held to maturity securities in unrealized loss positions for 12 months or more \| Security	13
State and municipal
Schedule of Investments [Line Items]
Securities Available for sale, Less than 12 Months: Fair Value	$ 1,042	4,933
Securities Available for sale, Less than 12 Months: Unrealized Losses	8	94
Securities Available for sale, 12 Months or More: Fair Value	1,950	0
Securities Available for sale, 12 Months or More: Unrealized Losses	34	0
Securities Available for sale, Total: Fair Value	2,992	4,933
Securities Available for sale, Total Unrealized Loss	$ 42	94
Available for sale securities in unrealized loss positions \| Security	14
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	3.00%
Available for sale securities in unrealized loss positions for 12 months or more \| Security	8
Corporate bonds
Schedule of Investments [Line Items]
Securities Available for sale, Total Unrealized Loss	$ 0	0
CRA mutual fund
Schedule of Investments [Line Items]
Securities Available for sale, Less than 12 Months: Fair Value	0	1,035
Securities Available for sale, Less than 12 Months: Unrealized Losses	0	9
Securities Available for sale, 12 Months or More: Fair Value	1,035	0
Securities Available for sale, 12 Months or More: Unrealized Losses	9	0
Securities Available for sale, Total: Fair Value	1,035	1,035
Securities Available for sale, Total Unrealized Loss	$ 9	9
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	1.00%
Stock In Other Banks
Schedule of Investments [Line Items]
Securities Available for sale, Total Unrealized Loss	$ 0	$ 0

Available-for-sale, Securities in Unrealized Loss Positions, Unrealized Losses as Percentage of Amortized Cost

No definition available.

Name:	acnb_AvailableforsaleSecuritiesinUnrealizedLossPositionsUnrealizedLossesasPercentageofAmortizedCost
Namespace Prefix:	acnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Held-to-maturity, Number of Securities in Continuous Loss Positions for 12 Months or More

+ References

No definition available.

+ Details

Name:	acnb_HeldtomaturityNumberofSecuritiesinContinuousLossPositionsfor12MonthsorMore
Namespace Prefix:	acnb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Held-to-maturity, Securities in Unrealized Loss Positions, Unrealized Losses as Percentage of Amortized Cost

+ References

No definition available.

+ Details

Name:	acnb_HeldtomaturitySecuritiesinUnrealizedLossPositionsUnrealizedLossesasPercentageofAmortizedCost
Namespace Prefix:	acnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity that have been in a continuous loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- 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.

+ Details

Name:	us-gaap_ScheduleOfInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=acnb_CraMutualFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=acnb_StockInOtherBanksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: