Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts receivable, trade, allowance for doubtful accounts	$ 2,599	$ 3,092
Property and equipment, accumulated depreciation	$ 142,815	$ 132,745
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	45,000,000	45,000,000
Preferred stock, shares issued	0	0
Treasury stock, shares	627,818	194,060
Class A [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	360,000,000	360,000,000
Common stock shares issued	58,798,794	58,628,764
Common stock, shares outstanding	58,170,976	58,434,704
Class B [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	90,000,000	90,000,000
Common stock shares issued	13,534,408	13,534,408
Common stock, shares outstanding	13,534,408	13,534,408