Related Party Transactions (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended						3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2012 Transactions with Lovell Minnick Partners [Member]	Dec. 31, 2012 Transactions with Lovell Minnick Partners [Member]	Dec. 31, 2011 Transactions with Lovell Minnick Partners [Member]	Dec. 31, 2010 Transactions with Lovell Minnick Partners [Member]	Dec. 31, 2012 Transactions with Vestar Capital Partners [Member]	Dec. 31, 2011 Transactions with Vestar Capital Partners [Member]	Dec. 31, 2010 Transactions with Vestar Capital Partners [Member]	Mar. 31, 2012 Follow On Offering Q1 2012 [Member] Transactions with Vestar Capital Partners [Member]	Mar. 31, 2012 Follow On Offering Q1 2012 [Member] Transactions with Noah Gottdiener [Member]	Mar. 31, 2012 Follow On Offering Q1 2012 [Member] Transactions with Harvey Krueger [Member]	Sep. 30, 2012 Follow On Offering Q3 2012 [Member] Transactions with Vestar Capital Partners [Member]	Sep. 30, 2012 Follow On Offering of Selling Shareholders [Member] Follow On Offering Q3 2012 [Member]
Related Party Transaction [Line Items]
Partners' Capital Account, Units, Redeemed	4,407			1,522							2,185
Distributions for taxes	$ 2,383	$ 4,812	$ 7,081		$ 379	$ 1,485	$ 1,890	$ 1,216	$ 2,058	$ 2,618
Other distributions	1,699	3,635	2,752		200	1,157	831	876	1,608	1,156
Payments Pursuant to the Tax Receivable Agreement	6,033	5,536	4,267		0	1,370	1,145	1,901	1,848	1,629
Distributions and Other Payments To Noncontrolling Unitholders	4,082	8,447	9,833		579	4,012	3,866	3,993	5,514	5,403
Redemption of New Class A Units, value per unit				13.38							13.38
Redemption of New Class A Units, value	58,972			20,363							29,231	65	52
Exchange of New Class A Units to shares of Class A common stock, shares	0	0	0	2,094
Corresponding Shares of Class B Common Stock Cancelled				2,094
Revenue from Related Parties					$ 106
Stock Sold by Affiliate, Shares														1,878	3,083
Stock Sold by Affiliates, Value Per Share															$ 13.25