LONG-TERM DEBT AND FINANCING ARRANGEMENTS - Assets securing notes (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Arrangements
Total assets securing notes payable or held under finance leases	$ 351,832	$ 353,093
Less accumulated depreciation and amortization	126,398	115,424
Net assets securing notes payable or held under finance leases	225,434	237,669
Revenue equipment
Financing Arrangements
Total assets securing notes payable or held under finance leases	325,562	326,823
Service, office, and other equipment
Financing Arrangements
Total assets securing notes payable or held under finance leases	$ 26,270	$ 26,270