CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 2,894,000	$ 3,091,000
Total assets of variable interest entities	113,000,000	107,900,000
Total liabilities of variable interest entities	$ 13,000,000	$ 7,900,000
Class A Common Stock
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized	500,000,000	500,000,000
Common Stock, shares issued	52,878,992	52,332,012
Common Stock, shares outstanding	52,878,992	52,332,012
Class B Common Stock
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized	140,000,000	140,000,000
Common Stock, shares issued	28,846,259	28,933,859
Common Stock, shares outstanding	28,846,259	28,933,859