CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 12,345	$ 11,228
Class A common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Class A common stock, shares authorized	199,300	199,300
Class A common stock, shares issued	100,320	99,301
Class A common stock, shares outstanding	85,188	84,395
Treasury stock, at cost, shares held	15,132	14,906