LONG-TERM DEBT AND FINANCING ARRANGEMENTS - Assets securing notes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Arrangements
Total assets securing notes payable or held under finance leases	$ 353,093	$ 293,799
Less accumulated depreciation and amortization	115,424	71,405
Net assets securing notes payable or held under finance leases	$ 237,669	222,394
Weighted-average interest rate (as a percent)	2.90%
Interest paid, net of capitalized interest	$ 11,300	10,900	$ 8,700
Capitalized interest	300	200
Revenue equipment
Financing Arrangements
Total assets securing notes payable or held under finance leases	326,823	265,315
Software
Financing Arrangements
Total assets securing notes payable or held under finance leases		2,140
Service, office, and other equipment
Financing Arrangements
Total assets securing notes payable or held under finance leases	$ 26,270	$ 26,344