PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Multiemployer Plans) (Details) $ in Thousands		1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Aug. 01, 2018	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) plan	Dec. 31, 2017
Multiemployer Plans
Multiemployer pension fund withdrawal liability charge (pre-tax)			$ 37,922	$ 37,922
Multiemployer pension plans | Asset-Based
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%
Multiemployer pension plans | Central States Pension Plan | Asset-Based
Multiemployer Plans
Approximate proportion of multiemployer pension plan contributions (as a percent)				50.00%
Multiemployer pension plans | New England Pension Fund | Asset-Based
Multiemployer Plans
Approximate proportion of multiemployer pension plan contributions (as a percent)					3.00%
Multiemployer pension fund withdrawal liability charge (pre-tax)		$ 37,900
Discount rate (as a percent)		4.50%	4.50%	4.50%
Initial lump sum cash payment liability		$ 15,100	$ 15,100	$ 15,100
Withdrawal liability monthly payments period (in years)		23 years
Aggregate present value of monthly payments		$ 22,800	$ 22,800	$ 22,800
Multiemployer pension plans | New England Pension Fund | Asset-Based | Subsequent Event
Multiemployer Plans
Contribution rate frozen period (in years)	10 years