CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CURRENT ASSETS:
Accounts receivable, allowance	$ 25,174	$ 18,107
PROPERTY, EQUIPMENT AND FACILITIES - accumulated depreciation and amortization	176,300	172,408
OTHER ASSETS:
Noncurrent receivables, allowance	$ 3,465	$ 2,260
STOCKHOLDERS' EQUITY:
Common stock, par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	26,476,329	25,231,710
Common stock, shares outstanding (in shares)	26,476,329	25,231,710
Treasury stock, shares (in shares)	5,910,541	5,910,541
Series A Convertible Preferred Stock [Member]
SERIES A CONVERTIBLE PREFERRED STOCK
Preferred stock, par value (in dollars per share)	$ 0	$ 0
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	12,700	0
Preferred stock, shares outstanding (in shares)	12,700	0