Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable, trade, allowance for doubtful accounts	$ 1,304	$ 1,378
Property and equipment, accumulated depreciation	$ 152,901	$ 147,084
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	863,875	656,013
Common 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	61,278,811	61,247,043
Common stock, shares outstanding	60,414,936	60,591,030
Common 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	11,784,408	11,784,408
Common stock, shares outstanding	11,784,408	11,784,408