Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Allowance on trade accounts receivable	$ 6,163	$ 5,161
Allowance on other receivables	6,306	5,523
Accumulated depreciation and amortization on operating property	$ 210,018	$ 190,374
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized shares	160,000,000	160,000,000
Common stock, issued shares	67,715,290	67,585,675
Treasury stock, shares	25,749,493	25,747,541