Securities - Narrative (Details) $ in Thousands	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Carrying value of securities pledged as collateral \| $	$ 149,780	$ 157,601
U.S. Government and agencies
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities in unrealized loss positions	65
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	6.00%
Available for sale securities in unrealized loss positions for 12 months or more	35
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities in unrealized loss positions	10
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	6
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities in unrealized loss positions	35
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	8.00%
Available for sale securities in unrealized loss positions for 12 months or more	3
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity securities in unrealized loss positions	33
Held to maturity securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	4.00%
Held to maturity securities in unrealized loss positions for 12 months or more	13
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities in unrealized loss positions	13
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	9.00%
Available for sale securities in unrealized loss positions for 12 months or more	8
Corporate bond
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities in unrealized loss positions, percentage of amortized cost (individually did not exceed)	1.00%

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

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ 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

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

- 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_ScheduleOfHeldToMaturitySecuritiesLineItems
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_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: