CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
CURRENT ASSETS:
Accounts receivable, allowance	$ 20,661	$ 18,107
PROPERTY, EQUIPMENT AND FACILITIES - accumulated depreciation and amortization	175,183	172,408
OTHER ASSETS:
Noncurrent receivables, allowance	$ 2,932	$ 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)	32,386,438	31,142,251
Common stock, shares outstanding (in shares)	32,386,438	31,142,251
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	12,700
Preferred stock, shares outstanding (in shares)	12,700	12,700