LONG-TERM DEBT AND FINANCING ARRANGEMENTS - Summary (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Long-term debt obligations
Long-term debt	$ 237,719	$ 284,224
Less current portion	66,644	67,105
Long-term debt, less current portion	171,075	217,119
Payments under long-term debt obligations
Due in one year or less	71,778
Due after one year through two years	57,710
Due after two years through three years	42,571
Due after three years through four years	72,345
Due after four years through five years	4,567
Due after five years	124
Total payments	249,095
Less amounts representing interest	11,376
Long-term debt	237,719	284,224
Credit Facility
Long-term debt obligations
Long-term debt	$ 50,000	70,000
Interest rate (as a percent)	1.20%
Payments under long-term debt obligations
Due in one year or less	$ 637
Due after one year through two years	784
Due after two years through three years	1,049
Due after three years through four years	50,296
Total payments	52,766
Less amounts representing interest	2,766
Long-term debt	50,000	70,000
Credit Facility | Interest rate swap agreement, maturing on June 30, 2022
Long-term debt obligations
Amount of borrowings covered by the interest rate swap	$ 50,000	$ 50,000
Effective fixed interest rate on hedged borrowings (as a percent)	3.12%	3.12%
Notes payable
Long-term debt obligations
Long-term debt	$ 187,715	$ 214,216
Weighted-average interest rate (as a percent)	2.90%
Payments under long-term debt obligations
Due in one year or less	$ 71,137
Due after one year through two years	56,926
Due after two years through three years	41,522
Due after three years through four years	22,049
Due after four years through five years	4,567
Due after five years	124
Total payments	196,325
Less amounts representing interest	8,610
Long-term debt	187,715	214,216
Finance lease obligations
Long-term debt obligations
Long-term debt	$ 4	8
Weighted-average interest rate (as a percent)	3.30%
Payments under long-term debt obligations
Due in one year or less	$ 4
Total payments	4
Long-term debt	$ 4	$ 8