CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 4,884	$ 4,039
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	65,762,612	65,460,014
Common stock, shares outstanding (in shares)	49,409,684	49,339,552
Non-vesting restricted shares (in shares)	715,263	841,610
Treasury shares (in shares)	16,352,928	16,120,462