OPERATING SEGMENT DATA - (Rev and Expenses) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
REVENUES
Revenues		$ 793,350	$ 720,368	$ 1,493,351	$ 1,371,456
OPERATING EXPENSES
Salaries, wages, and benefits		355,913	349,200	684,670	681,790
Fuel, supplies, and expenses		84,884	68,451	163,530	141,113
Depreciation and amortization		27,187	25,209	53,673	50,603
Multiemployer pension fund withdrawal liability charge		37,922		37,922
(Gain) loss on sale of property and equipment				(166)	(412)
Other		30,408	35,141	59,663	63,981
Restructuring costs		340	363	716	1,994
Total consolidated operating expenses		790,194	694,601	1,477,470	1,355,589
OPERATING INCOME
OPERATING INCOME		3,156	25,767	15,881	15,867
OTHER INCOME (COSTS)
Interest and dividend income		714	285	1,240	559
Interest and other related financing costs		(2,013)	(1,389)	(4,072)	(2,704)
Other, net		(1,123)	(528)	(3,324)	(2,234)
TOTAL OTHER INCOME (COSTS)		(2,422)	(1,632)	(6,156)	(4,379)
INCOME BEFORE INCOME TAXES		734	24,135	9,725	11,488
Asset-Based | Multiemployer pension plans | New England Pension Fund
OPERATING EXPENSES
Multiemployer pension fund withdrawal liability charge	$ 37,900
Operating Segments | Asset-Based
REVENUES
Revenues		559,239		1,041,354
OPERATING EXPENSES
Salaries, wages, and benefits		286,750	286,904	556,529	566,284
Fuel, supplies, and expenses		65,040	58,541	127,233	116,931
Operating taxes and licenses		11,910	12,191	23,666	24,014
Insurance		7,979	7,602	14,607	14,720
Communications and utilities		4,135	4,168	8,656	8,685
Depreciation and amortization		21,362	20,716	42,292	41,234
Rents and purchased transportation		63,253	53,189	109,386	99,615
Shared services		56,825	46,600	102,432	90,104
Multiemployer pension fund withdrawal liability charge		37,922		37,922
(Gain) loss on sale of property and equipment		(266)	25	(399)	(592)
Other		948	1,673	2,247	3,178
Restructuring costs			33		173
Total consolidated operating expenses		555,858	491,642	1,024,571	964,346
OPERATING INCOME
OPERATING INCOME		3,381	22,895	16,783	14,547
Operating Segments | ArcBest
REVENUES
Revenues		199,987		381,920
OPERATING EXPENSES
Purchased transportation		162,920	139,432	311,292	261,419
Supplies and expenses		3,538	3,742	6,768	7,412
Depreciation and amortization		3,597	3,230	7,005	6,496
Shared services		23,536	20,658	45,404	40,244
Other		2,546	2,873	4,427	5,338
Restructuring costs		143	65	152	875
Total consolidated operating expenses		196,280	170,000	375,048	321,784
OPERATING INCOME
OPERATING INCOME		3,707	5,929	6,872	7,021
Operating Segments | FleetNet
REVENUES
Revenues		46,792		94,551
OPERATING EXPENSES
Total consolidated operating expenses		45,763	35,754	92,001	74,971
OPERATING INCOME
OPERATING INCOME		1,029	747	2,550	1,768
Other and eliminations
REVENUES
Revenues		(12,668)		(24,474)
OPERATING EXPENSES
Total consolidated operating expenses		(7,707)	(2,795)	(14,150)	(5,512)
OPERATING INCOME
OPERATING INCOME		$ (4,961)	(3,804)	$ (10,324)	(7,469)
Calculated under Revenue Guidance in Effect before Topic 606
REVENUES
Revenues			720,368		1,371,456
Calculated under Revenue Guidance in Effect before Topic 606 | Operating Segments | Asset-Based
REVENUES
Revenues			514,537		978,893
Calculated under Revenue Guidance in Effect before Topic 606 | Operating Segments | ArcBest
REVENUES
Revenues			175,929		328,805
Calculated under Revenue Guidance in Effect before Topic 606 | Operating Segments | FleetNet
REVENUES
Revenues			36,501		76,739
Calculated under Revenue Guidance in Effect before Topic 606 | Other and eliminations
REVENUES
Revenues			$ (6,599)		$ (12,981)