Amount of pre-tax expenses associated with multiemployer pension fund withdrawal liability.

No definition available.

Name:	arcb_MultiemployerPensionFundWithdrawalLiabilityCharge
Namespace Prefix:	arcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the projected funded percentage of a specified multiemployer pension plan.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPensionPlanFundedPercentage
Namespace Prefix:	arcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of the entity's contributions to multiemployer pension plans that are made to the specified plan.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPensionPlansPercentageOfContributions
Namespace Prefix:	arcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Discount rate applied to the undiscounted amount of future payments due to determine the present value of the multiemployer plan withdrawal liability recorded as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlanWithdrawalLiabilityDiscountRatePercent
Namespace Prefix:	arcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The threshold funded percentage of plans, below which a higher maximum period for insolvency is used to determine "critical and declining" status.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansCriticalAndDecliningStatusThresholdFundedPercentageForGreaterInsolvencyPeriod
Namespace Prefix:	arcb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The threshold ratio of inactive participants to active participants, above which a higher maximum period for insolvency is used to determine "critical and declining" status.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansCriticalAndDecliningStatusThresholdRatioOfInactiveToActiveParticipantsForGreaterInsolvencyPeriod
Namespace Prefix:	arcb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum period within which a plan in "critical and declining" status is expected to become insolvent, unless additional criteria relating to ratio of inactive to active participants or funded percentage are applicable.

+ References

No definition available.

+ Details

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

- Definition

The maximum period within which a plan in "critical and declining" status is expected to become insolvent, if additional criteria relating to ratio of inactive to active participants or funded percentage are applicable.

+ References

No definition available.

+ Details

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

- Definition

The period of time over which monthly payments have to be made for withdrawal liability settlement for multiemployer plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of multiemployer plans to which the reporting entity currently contributes.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansNumber
Namespace Prefix:	arcb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow for payment of withdrawal liability from multiemployer plan

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansPaymentOfWithdrawalObligation
Namespace Prefix:	arcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate present value of the monthly payments for the multiemployer plan withdrawal liability.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansWithdrawalLiabilityMonthlyPaymentsAggregatePresentValue
Namespace Prefix:	arcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of the obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

No definition available.

+ Details

Name:	arcb_MultiemployerPlansWithdrawalObligationFairValue
Namespace Prefix:	arcb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of the obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

No definition available.

+ Details

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

- Details

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- Details

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- Details

Name:	us-gaap_MultiemployerPlanNameAxis=arcb_CentralStatesPensionPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_MultiemployerPlanNameAxis=arcb_NewEnglandPensionFundMember
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Data Type:	na
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- Details

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- Details

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PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Multiemployer Plans) (Details) - Multiemployer pension plans - Asset-Based $ in Millions			3 Months Ended
	Jan. 01, 2019	Jun. 30, 2018 USD ($)	Mar. 31, 2020 USD ($) plan	Sep. 30, 2018 USD ($)	Jan. 01, 2018
Multiemployer Plans
Number of multiemployer plans to which ABF Freight currently contributes \| plan			25
Maximum projected time to insolvency for plans in "critical and declining" status	14 years
Maximum projected time to insolvency for plans in "critical and declining" status if additional criteria apply	19 years
Threshold ratio of inactive to active participants for greater insolvency period to determine "critical and declining" status	2
Threshold funded percentage for greater insolvency period to determine "critical and declining" status	80.00%
Central States Pension Plan
Multiemployer Plans
Approximate proportion of multiemployer pension plan contributions (as a percent)			50.00%
Actuarially certified projected funded percentage of multiemployer pension plan					27.20%
New England Pension Fund
Multiemployer Plans
Multiemployer pension fund withdrawal liability charge (pre-tax)		$ 37.9
Initial lump sum payment of withdrawal liability				$ 15.1
Aggregate present value of monthly payments		$ 22.8
Withdrawal liability monthly payments period (in years)				23 years
Outstanding withdrawal liability			$ 21.9
Fair value of withdrawal obligation			$ 27.0
Discount rate using 20-year U.S. Treasury (as a percent)			2.30%
New England Pension Fund \| Accrued expenses
Multiemployer Plans
Outstanding withdrawal liability			$ 0.6
New England Pension Fund \| Other long-term liabilities
Multiemployer Plans
Outstanding withdrawal liability			$ 21.3