CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
CURRENT ASSETS:
Accounts receivable, allowance	$ 16,195	$ 15,590
PROPERTY, EQUIPMENT AND FACILITIES - accumulated depreciation and amortization	173,923	171,109
OTHER ASSETS:
Noncurrent receivables, allowance	$ 1,575	$ 1,403
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)	31,142,251	30,552,333
Common stock, shares outstanding (in shares)	31,142,251	30,552,333
Treasury stock, shares (in shares)	5,910,541	5,910,541