CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Allowance on trade accounts receivable	$ 7,379	$ 6,591
Allowance on other receivables	4,727	5,384
Accumulated depreciation and amortization on operating property	$ 150,687	$ 145,804
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized shares	160,000,000	160,000,000
Common stock, issued shares	66,851,672	66,602,486
Treasury stock, shares	20,202,662	19,882,289