Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accounts receivable, trade, allowance for doubtful accounts	$ 1,398	$ 1,378
Property and equipment, accumulated depreciation	$ 164,349	$ 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	891,696	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	61,689,396	61,247,043
Common stock, shares outstanding	60,797,700	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	11,484,408	11,784,408
Common stock, shares outstanding	11,484,408	11,784,408