Consolidated Balance Sheets Consolidated Balance Sheets (Parethetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 4,167,000	$ 3,593,000
Accumulated depreciation of property assets	$ 525,673,000	$ 522,101,000
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,681,559	109,519,369
Treasury stock - shares at cost	56,369,752,000	56,369,752