Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Receivables, allowance for doubtful accounts	$ 16,325	$ 15,905
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)	55,696,356	55,679,207
Treasury stock, shares (in shares)	39,369,282	39,386,431