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EMPLOYEE BENEFIT PLANS (Multiemployer Plans) (Details) $ in Thousands				1 Months Ended	3 Months Ended		12 Months Ended
	Aug. 01, 2018	Aug. 01, 2017	Aug. 01, 2016	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) plan	Jan. 31, 2018	Dec. 31, 2017 USD ($)	Jan. 31, 2017	Dec. 31, 2016 USD ($)	Jan. 01, 2017	Jan. 01, 2016
Multiemployer Plans
Multiemployer pension fund withdrawal liability charge (pre-tax)						$ 37,900	$ 37,922
Asset-Based
Multiemployer Plans
Minimum funded percentage of plans in green zone							80.00%
Asset-Based | Multiemployer pension plans
Multiemployer Plans
Number of multiemployer plans to which ABF Freight currently contributes | plan							25
Maximum funded percentage of plans in yellow zone							80.00%
Maximum funded percentage of plans in red zone							65.00%
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%
Percentage of contributions to the multiemployer pension plans that are in critical and declining status							62.00%
Percentage of contributions to the multiemployer pension plans that are in critical status							2.00%
Percentage of contributions to the multiemployer pension plans that are in endangered status							4.00%
Total contributions to multiemployer plans							$ 167,249		$ 158,416		$ 154,139
Percentage increase in contribution rate for time worked related to benefit costs		1.30%	0.30%
Asset-Based | Multiemployer pension plans | Central States Pension Plan
Multiemployer Plans
Total contributions to multiemployer plans							$ 74,177		$ 78,230		$ 77,891
Threshold percentage of the entity's contributions relative to total fund contributions, which was exceeded during the period									5.00%		5.00%
Approved increase in contribution (as a percent)		0.0	0.0
Maximum projected time to insolvency without legislative action							10 years
Percentage of contributions to multiemployer pension plan							50.00%		50.00%		50.00%
Actuarially certified projected funded percentage of multiemployer pension plan												37.80%	42.10%
Asset-Based | Multiemployer pension plans | Central Pennsylvania Teamsters Pension Plan
Multiemployer Plans
Total contributions to multiemployer plans							$ 13,393		$ 13,391		$ 13,381
Asset-Based | Multiemployer pension plans | Western Conference of Teamsters Pension Plan
Multiemployer Plans
Total contributions to multiemployer plans							25,268		26,320		25,075
Approved increase in contribution (as a percent)		0.0	0.0
Asset-Based | Multiemployer pension plans | Local 710 Pension Fund
Multiemployer Plans
Total contributions to multiemployer plans							9,929		10,054		9,670
Threshold percentage of the entity's contributions relative to total fund contributions, which was exceeded during the period								5.00%		5.00%
Asset-Based | Multiemployer pension plans | All Other Pension Plans in Aggregate
Multiemployer Plans
Total contributions to multiemployer plans							24,392		25,395		23,718
Asset-Based | Multiemployer pension plans | New England Pension Fund
Multiemployer Plans
Total contributions to multiemployer plans							20,090		5,026		4,404
Contributions to multiemployer plans related to pension fund withdrawal liability							15,700
Contribution rate frozen period (in years)	10 years
Multiemployer pension fund withdrawal liability charge (pre-tax)				$ 37,900
Initial lump sum payment of withdrawal liability					$ 15,100
Withdrawal liability monthly payments period (in years)					23 years
Aggregate present value of monthly payments					$ 22,800
Outstanding withdrawal liability							22,600
Fair value of withdrawal obligation							$ 23,400
Discount rate using 20-year U.S. Treasury (as a percent)							4.10%
Asset-Based | Multiemployer pension plans | New England Pension Fund | Accrued expenses
Multiemployer Plans
Outstanding withdrawal liability							$ 600
Asset-Based | Multiemployer pension plans | New England Pension Fund | Other long-term liabilities
Multiemployer Plans
Outstanding withdrawal liability							$ 22,000
Asset-Based | Multiemployer pension plans | 707 Pension Fund
Multiemployer Plans
Percentage of contributions to multiemployer pension plan							1.00%
Asset-Based | Multiemployer pension plans | New York State Pension Fund
Multiemployer Plans
Percentage of contributions to multiemployer pension plan							2.00%
Asset-Based | Multiemployer health and welfare plans
Multiemployer Plans
Number of multiemployer plans to which ABF Freight currently contributes | plan							38
Total contributions to multiemployer plans							$ 162,100		$ 162,200		$ 153,300
Percentage increase in contribution rate for time worked related to benefit costs	3.90%	3.70%	3.50%
Asset-Based | Unionized employees concentration risk | Number of employees
Multiemployer Plans
Percentage of Asset-Based segment employees covered under collective bargaining agreement with the IBT							82.00%

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

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

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Represents the maximum funded percentage of plans in the red zone.

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Represents the maximum funded percentage of plans in the yellow zone.

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Represents the minimum funded percentage of plans in the yellow zone.

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

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Represents the percentage of the entity's contributions to multiemployer pension plans that are made to plans in critical and declining status.

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Represents the percentage of the entity's contributions to multiemployer pension plans that are made to plans in critical status.

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Approved increase in employer's contractual contribution rate to multiemployer plan by Negotiating Committee of Plan.

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Represents the threshold percentage of the entity's contributions to the plan relative to total contributions received by the plan, which was exceeded during the period, triggering disclosure of the plan year.

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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.

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The period for which contribution rate is frozen under multiemployer plan agreement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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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.

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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.

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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.

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The maximum period within which a plan is expected to become insolvent without legislation action.

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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.

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

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Amount of cash outflow for initial lumpsum payment of withdrawal liability from multiemployer plan.

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

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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.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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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.

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