Condensed Statements Of Financial Condition, Parent Company Only (Parenthetical) (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Aug. 19, 2011
Condensed Financial Statements, Captions [Line Items]
Treasury Stock at Cost, shares	5,463,515	3,072,958
Class A
Condensed Financial Statements, Captions [Line Items]
Common Stock, par value	$ 0.01	$ 0.01	$ 0.01
Common Stock, shares authorized	1,000,000,000	1,000,000,000
Common Stock, shares issued	35,040,501	31,014,265
Common Stock, shares outstanding	29,576,986	27,941,307
Class B
Condensed Financial Statements, Captions [Line Items]
Common Stock, par value	$ 0.01	$ 0.01
Common Stock, shares authorized	1,000,000	1,000,000
Common Stock, shares issued	43	47
Common Stock, shares outstanding	43	47
Parent Company
Condensed Financial Statements, Captions [Line Items]
Treasury Stock at Cost, shares	5,463,515	3,072,958
Parent Company | Class A
Condensed Financial Statements, Captions [Line Items]
Common Stock, par value	$ 0.01	$ 0.01
Common Stock, shares authorized	1,000,000,000	1,000,000,000
Common Stock, shares issued	35,040,501	31,014,265
Common Stock, shares outstanding	29,576,986	27,941,307
Parent Company | Class B
Condensed Financial Statements, Captions [Line Items]
Common Stock, par value	$ 0.01	$ 0.01
Common Stock, shares authorized	1,000,000	1,000,000
Common Stock, shares issued	43	47
Common Stock, shares outstanding	43	47