OPERATING SEGMENT DATA - Operating Expenses by Category (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 EXPENSES
Salaries, wages, and benefits									$ 1,368,588	$ 1,408,409	$ 1,398,348
Rents, purchased transportation, and other costs of services									974,835	934,958	989,006
Fuel, supplies, and expenses									250,221	316,047	325,126
Depreciation and amortization									118,391	112,466	108,635
Other									129,850	126,146	123,998
Asset impairment					$ 26,500					26,514
Multiemployer pension fund withdrawal liability charge											37,922
Restructuring costs											1,655
Total consolidated operating expenses	$ 786,162	$ 755,198	$ 606,945	$ 693,580	$ 728,647	$ 756,355	$ 736,290	$ 703,248	$ 2,841,885	$ 2,924,540	$ 2,984,690