Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Class A
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	180,000,000	180,000,000
Common stock, shares issued	53,010,983	52,717,553
Common stock, shares outstanding	40,521,990	40,257,656
Treasury stock, shares	12,488,993	12,459,897
Class B
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	30,000,000	30,000,000
Common stock, shares issued	2,112,048	2,192,473
Common stock, shares outstanding	2,112,048	2,192,473