Loans - Schedule of Allowance for Loan Losses and Recorded Investment in Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	$ 20,226	$ 13,835
Charge-offs	(55)	(2,068)
Recoveries	16	85
Provisions (credits)	50	4,000
Allowance for Loan Losses, Ending Balance	20,237	15,852
Allowance for Loan Losses, Ending Balance	20,226	15,852	$ 20,237	$ 20,226	$ 15,852
Allowance for loan losses: Ending balance: individually evaluated for impairment			2,106	1,382	78
Allowance for loan losses: Ending balance: collectively evaluated for impairment			18,131	18,844	15,774
Total Loans Receivable			1,610,718	1,637,784	1,606,039
Loans receivables: Ending balance: individually evaluated for impairment			11,345	10,721	8,584
Loans Receivable: Ending balance: collectively evaluated for impairment			1,599,373	1,627,063	1,597,455
Commercial and Industrial
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	4,037	2,400
Charge-offs	(18)	(2,017)
Recoveries	8	70
Provisions (credits)	476	2,193
Allowance for Loan Losses, Ending Balance	4,503	2,646
Allowance for Loan Losses, Ending Balance	4,037	2,646	4,503	4,037	2,646
Allowance for loan losses: Ending balance: individually evaluated for impairment			1,954	1,224	36
Allowance for loan losses: Ending balance: collectively evaluated for impairment			2,549	2,813	2,610
Total Loans Receivable			315,710	320,154	208,859
Loans receivables: Ending balance: individually evaluated for impairment			2,739	2,031	58
Loans Receivable: Ending balance: collectively evaluated for impairment			312,971	318,123	208,801
Commercial Real Estate
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	9,569	6,693
Charge-offs	0	0
Recoveries	0	6
Provisions (credits)	(29)	812
Allowance for Loan Losses, Ending Balance	9,540	7,511
Allowance for Loan Losses, Ending Balance	9,540	7,511	9,540	9,569	7,511
Allowance for loan losses: Ending balance: individually evaluated for impairment			152	158	42
Allowance for loan losses: Ending balance: collectively evaluated for impairment			9,388	9,411	7,469
Total Loans Receivable			759,335	744,627	763,415
Loans receivables: Ending balance: individually evaluated for impairment			8,505	8,589	8,357
Loans Receivable: Ending balance: collectively evaluated for impairment			750,830	736,038	755,058
Commercial Real Estate Construction
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	503	298
Charge-offs	0	0
Recoveries	0	0
Provisions (credits)	40	(67)
Allowance for Loan Losses, Ending Balance	543	231
Allowance for Loan Losses, Ending Balance	543	231	543	503	231
Allowance for loan losses: Ending balance: individually evaluated for impairment			0	0	0
Allowance for loan losses: Ending balance: collectively evaluated for impairment			543	503	231
Total Loans Receivable			50,132	53,902	44,198
Loans receivables: Ending balance: individually evaluated for impairment			0	0	0
Loans Receivable: Ending balance: collectively evaluated for impairment			50,132	53,902	44,198
Residential Mortgage
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	3,395	2,555
Charge-offs	0	0
Recoveries	0	0
Provisions (credits)	(29)	146
Allowance for Loan Losses, Ending Balance	3,366	2,701
Allowance for Loan Losses, Ending Balance	3,366	2,701	3,366	3,395	2,701
Allowance for loan losses: Ending balance: individually evaluated for impairment			0	0	0
Allowance for loan losses: Ending balance: collectively evaluated for impairment			3,366	3,395	2,701
Total Loans Receivable			375,941	401,340	456,436
Loans receivables: Ending balance: individually evaluated for impairment			101	101	101
Loans Receivable: Ending balance: collectively evaluated for impairment			375,840	401,239	456,335
Home Equity Lines of Credit
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	693	619
Charge-offs	0	0
Recoveries	0	0
Provisions (credits)	(64)	(14)
Allowance for Loan Losses, Ending Balance	629	605
Allowance for Loan Losses, Ending Balance	629	605	629	693	605
Allowance for loan losses: Ending balance: individually evaluated for impairment			0	0	0
Allowance for loan losses: Ending balance: collectively evaluated for impairment			629	693	605
Total Loans Receivable			97,258	104,610	118,603
Loans receivables: Ending balance: individually evaluated for impairment			0	0	68
Loans Receivable: Ending balance: collectively evaluated for impairment			97,258	104,610	118,535
Consumer
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	648	650
Charge-offs	(37)	(51)
Recoveries	8	9
Provisions (credits)	(11)	15
Allowance for Loan Losses, Ending Balance	608	623
Allowance for Loan Losses, Ending Balance	648	623	608	648	623
Allowance for loan losses: Ending balance: individually evaluated for impairment			0	0	0
Allowance for loan losses: Ending balance: collectively evaluated for impairment			608	648	623
Total Loans Receivable			12,342	13,151	14,528
Loans receivables: Ending balance: individually evaluated for impairment			0	0	0
Loans Receivable: Ending balance: collectively evaluated for impairment			12,342	13,151	14,528
Unallocated
Allowance for Loan Losses [Roll Forward]
Allowance for Loan Losses, Beginning Balance	1,381	620
Charge-offs	0	0
Recoveries	0	0
Provisions (credits)	(333)	915
Allowance for Loan Losses, Ending Balance	1,048	1,535
Allowance for Loan Losses, Ending Balance	$ 1,381	$ 1,535	1,048	1,381	1,535
Allowance for loan losses: Ending balance: individually evaluated for impairment			0	0	0
Allowance for loan losses: Ending balance: collectively evaluated for impairment			1,048	1,381	1,535
Total Loans Receivable			0	0	0
Loans receivables: Ending balance: individually evaluated for impairment			0	0	0
Loans Receivable: Ending balance: collectively evaluated for impairment			$ 0	$ 0	$ 0

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

Name:	us-gaap_ProvisionForLoanLossesExpensed
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=acnb_HomeEquityLinesOfCreditPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: