Noncontrolling Interest (Details)				12 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Jun. 30, 2007	Dec. 31, 2011 Private Placement [Member]	Sep. 30, 2012 Common Class B [Member]	Dec. 30, 2007 Common Class B [Member]	Dec. 31, 2011 Common Class B [Member]	Dec. 31, 2010 Common Class B [Member]	Dec. 31, 2009 Common Class B [Member]	Dec. 31, 2008 Common Class B [Member]
LLC Membership Units and Class B common shares outstanding at initial public offering	5,483,773	5,270,848	897,903		795,479		931,154
LLC Membership Units and Class B common shares exchanged					(135,675)	(28,060)	(342,916)	(16,830)	(131,684)	(365,722)
Remaining LLC Membership Units and Class B common shares				918,463	795,479