Consolidated Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Allowance for doubtful trade accounts receivable	$ 8,457	$ 8,613
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	80,533,536	80,533,536
Common stock, shares outstanding	69,888,102	70,441,973
Treasury stock, shares	10,645,434	10,091,563