OPERATING SEGMENT DATA - Operating Expenses by Category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
OPERATING EXPENSES
Salaries, wages, and benefits	$ 359,395	$ 344,946
Rents, purchased transportation, and other costs of services	309,338	217,028
Fuel, supplies, and expenses	73,149	71,773
Depreciation and amortization	30,354	29,013
Other	24,786	30,820
Total consolidated operating expenses	$ 797,022	$ 693,580