OPERATING SEGMENT DATA - Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING INCOME (LOSS)
Operating income (loss)	$ 30,252	$ 39,782	$ 20,425	$ 7,819	$ (11,229)	$ 31,208	$ 35,200	$ 8,591	$ 98,278	$ 63,770	$ 109,098
OTHER INCOME (COSTS)
Interest and dividend income									3,616	6,453	3,914
Interest and other related financing costs									(11,697)	(11,467)	(9,468)
Other, net									2,299	(7,285)	(19,158)
TOTAL OTHER INCOME (COSTS)	$ (53)	$ (604)	$ 309	$ (5,434)	$ (798)	$ (7,866)	$ (1,640)	$ (1,995)	(5,782)	(12,299)	(24,712)
INCOME BEFORE INCOME TAXES									92,496	51,471	84,386
Operating Segments | Asset Based
OPERATING INCOME (LOSS)
Operating income (loss)									98,865	102,061	103,862
Operating Segments | ArcBest
OPERATING INCOME (LOSS)
Operating income (loss)									9,655	(20,189)	23,588
Operating Segments | FleetNet
OPERATING INCOME (LOSS)
Operating income (loss)									3,367	4,806	4,385
Other and eliminations
OPERATING INCOME (LOSS)
Operating income (loss)									$ (13,609)	$ (22,908)	$ (22,737)