Consolidated Balance Sheets (Parenthetical) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Common Class A [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares issued	97,478,074	97,436,947
Common stock, shares outstanding	72,679,123	73,602,866
Class A Treasury common stock, shares	24,798,951	23,834,081
Common Class B [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	10,838,731	10,838,731
Common stock, shares outstanding	10,838,731	10,838,731