Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Allowance for doubtful accounts	$ 3,244	$ 3,614
Accumulated depreciation, property assets	$ 508,629	$ 482,448
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,519,369	109,441,911
Treasury stock - shares at cost	56,369,752	56,369,752