Consolidated Balance Sheets Parenthetical (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Allowance for doubtful accounts	$ 4,605	$ 4,361
Accumulated depreciation	$ 808,562	$ 779,449
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	4,500,000	4,500,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	48,437,917