Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Receivables, allowance for doubtful accounts	$ 14,608	$ 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)	56,660,639	56,679,968
Treasury stock, shares (in shares)	38,404,999	38,385,670