OPERATING SEGMENT DATA - Operating expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
OPERATING EXPENSES
Salaries, wages, and benefits	$ 359,395	$ 344,946
Fuel, supplies, and expenses	73,149	71,773
Depreciation and amortization	30,354	29,013
Gain on sale of property and equipment	(8,635)	(2,130)
Other	24,786	30,820
Total consolidated operating expenses	797,022	693,580
Operating Segments | Asset Based
OPERATING EXPENSES
Salaries, wages, and benefits	285,694	283,838
Fuel, supplies, and expenses	60,841	61,225
Operating taxes and licenses	12,248	12,794
Insurance	8,939	7,824
Communications and utilities	4,970	4,711
Depreciation and amortization	23,484	23,270
Rents and purchased transportation	75,588	55,770
Shared services	55,866	48,885
Gain on sale of property and equipment	(8,695)	(2,164)
Innovative technology costs	6,868	4,533
Other	434	1,787
Total consolidated operating expenses	526,237	502,473
Operating Segments | Asset Based | Land and structures (service centers)
OPERATING EXPENSES
Gain on sale of property and equipment	(8,600)
Operating Segments | ArcBest
OPERATING EXPENSES
Purchased transportation	210,995	137,182
Supplies and expenses	2,568	2,280
Depreciation and amortization	2,386	2,470
Shared services	26,072	21,727
Other	2,050	2,525
Total consolidated operating expenses	244,071	166,184
Operating Segments | FleetNet
OPERATING EXPENSES
Total consolidated operating expenses	58,140	51,399
Other and eliminations
OPERATING EXPENSES
Total consolidated operating expenses	$ (31,426)	$ (26,476)