CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Trade accounts receivables, allowance for doubtful accounts	$ 10,425	$ 15,312
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	110,547,000	112,524,000
Common stock, shares outstanding	110,547,000	112,524,000