OPERATING SEGMENT DATA - Capital expenditures, depreciation and amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING SEGMENT DATA
CAPITAL EXPENDITURES, GROSS	$ 105,051	$ 160,684	$ 137,675
DEPRECIATION AND AMORTIZATION EXPENSE	118,391	112,466	108,635
Amortization of intangibles	4,012	4,367	4,521
Operating Segments | Asset Based
OPERATING SEGMENT DATA
CAPITAL EXPENDITURES, GROSS	85,135	122,437	116,505
DEPRECIATION AND AMORTIZATION EXPENSE	94,326	89,798	85,951
Assets acquired through notes payable	61,800	67,600	86,800
Operating Segments | ArcBest
OPERATING SEGMENT DATA
CAPITAL EXPENDITURES, GROSS	1,258	3,909	5,174
DEPRECIATION AND AMORTIZATION EXPENSE	9,714	11,344	13,750
Amortization of intangibles	3,700	4,200	4,300
Operating Segments | FleetNet
OPERATING SEGMENT DATA
CAPITAL EXPENDITURES, GROSS	675	590	1,365
DEPRECIATION AND AMORTIZATION EXPENSE	1,622	1,341	1,140
Amortization of intangibles	200	200	200
Other and eliminations
OPERATING SEGMENT DATA
CAPITAL EXPENDITURES, GROSS	17,983	33,748	14,631
DEPRECIATION AND AMORTIZATION EXPENSE	$ 12,729	9,983	7,794
Assets acquired through notes payable		$ 23,200	$ 6,900