Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Receivables, allowance for doubtful accounts	$ 15,509	$ 12,221
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	160,000,000	160,000,000
Common stock, shares issued (in shares)	95,065,638	95,065,638
Common stock, shares outstanding (in shares)	56,223,839	56,679,968
Treasury stock, shares (in shares)	38,841,799	38,385,670