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