Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Accounts receivable, trade, allowance for doubtful accounts	$ 38	$ 931
Property and equipment, accumulated depreciation	$ 169,353	$ 164,865
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	1,135,939	898,447
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	61,728,148	61,692,561
Common stock, shares outstanding	60,592,209	60,794,114
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	11,484,408
Common stock, shares outstanding	11,484,408	11,484,408