CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
CURRENT ASSETS:
Accounts receivable, allowance	$ 15,721	$ 12,806
PROPERTY, EQUIPMENT AND FACILITIES - accumulated depreciation and amortization	169,026	163,946
OTHER ASSETS:
Noncurrent receivables, allowance	$ 1,240	$ 978
STOCKHOLDERS' EQUITY:
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)	0	0
Preferred stock, shares outstanding (in shares)	0	0
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)	30,552,333	30,624,407
Common stock, shares outstanding (in shares)	30,552,333	30,624,407
Treasury stock, shares (in shares)	5,910,541	5,910,541