Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 4,247	$ 4,167
Property assets, accumulated depreciation	$ 545,619	$ 525,673
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,879,035	109,681,559
Treasury stock - shares at cost	56,369,752	56,369,752