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Basis of Presentation and Nature of Operations (Details) - bank |  |  | 3 Months Ended
--- | --- | --- | ---
Jan. 11, 2020 | Jul. 01, 2017 | Mar. 31, 2021
Business Acquisition [Line Items] |  |  |
Number of community banking office locations, ACNB |  |  | 31
Adams, Cumberland, Franklin and York Counties, Pennsylvania |  |  |
Business Acquisition [Line Items] |  |  |
Number of community banking office locations, ACNB |  |  | 20
New Windsor |  |  |
Business Acquisition [Line Items] |  |  |
Number of community banking offices acquired |  | 6 |
Frederick County Bancorp, Inc. |  |  |
Business Acquisition [Line Items] |  |  |
Number of community banking offices acquired | 5 |  |

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- Definition Number of Community Banking Offices, Acquired + References No definition available. + Details
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Namespace Prefix: | acnb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of Community Banking Offices, Parent + References No definition available. + Details
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Namespace Prefix: | acnb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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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. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Details
Name: | srt_StatementGeographicalAxis=acnb_AdamsCumberlandFranklinAndYorkCountiesPennsylvaniaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=acnb_NewWindsorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=acnb_FrederickCountyBancorpInc.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |