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EMPLOYEE BENEFIT PLANS - Settlement distributions and expense (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Distributions and pension settlement expense
Pension settlement expense, pre-tax	$ 89
Supplemental Benefit Plan
Distributions and pension settlement expense
Pension settlement distributions	2,887
Pension settlement expense, pre-tax	$ 89
Period of delay for pension settlement distribution to key employees	6 months

X

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

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Namespace Prefix:	arcb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the period of delay provision for key employees as required by Section 409A of the Internal Revenue Code related to lump-sum termination payment.

+ References

No definition available.

+ Details

Name:	arcb_DefinedBenefitPlanLumpsumTerminationPaymentPeriodOfDelayProvisionForKeyEmployeesAsPerInternalRevenueCode
Namespace Prefix:	arcb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3A
-URI https://asc.fasb.org/extlink&oid=123453770&loc=SL108413299-114919

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)(7)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

+ Details

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

X

- Definition

Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(8)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: