Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 2,681	$ 3,593
Property assets, accumulated depreciation	$ 552,653	$ 522,101
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,671,676	109,519,369
Treasury stock - shares at cost	56,369,752	56,369,752