Condensed Consolidated Statements of Financial Condition (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets
Fixed assets, accumulated depreciation and amortization (in dollars)	$ 30,003	$ 23,852
Intangible assets, accumulated amortization (in dollars)	$ 22,945	$ 20,220
Stockholders' equity
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares authorized	500,000,000
Common Stock Class A
Stockholders' equity
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	123,740,112	119,393,640
Common stock, shares outstanding	112,447,892	114,047,637
Common stock, restricted shares	336,895	576,892
Treasury stock, shares	11,292,220	5,346,003
Common Stock Class B
Stockholders' equity
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	0	0
Common stock, shares outstanding	0	0