Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Class A
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	180,000,000	90,000,000
Common stock, shares issued	52,526,102	51,861,249
Common stock, shares outstanding	40,066,867	39,426,351
Treasury stock, shares	12,459,235	12,434,898
Class B
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	30,000,000	15,000,000
Common stock, shares issued	2,206,343	2,409,027
Common stock, shares outstanding	2,206,343	2,409,027