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Consolidated Balance Sheets Consolidated Balance Sheets (Parethetical) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Statement of Financial Position [Abstract]    
Allowance for doubtful accounts $ 4,883 $ 4,167
Accumulated depreciation of property assets $ 551,750 $ 525,673
Common stock - par value $ 0.01 $ 0.01
Common stock - shares authorized 250,000,000 250,000,000
Common stock - shares issued 109,909,504 109,681,559
Treasury stock - shares at cost 56,369,752 56,369,752