Consolidated Statements of Financial Condition (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 5.2	$ 7.1
Advisory fees receivable, allowance for doubtful accounts	0	0
Property and equipment, accumulated depreciation	$ 56.2	$ 54.8
Common stock, par value (usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	37,840,092	36,513,507
Common stock, shares outstanding	27,747,240	27,488,828
Contingent convertible preferred stock, par value (usd per share)	$ 0.01	$ 0.01
Contingent convertible preferred stock, shares authorized	10,000,000	10,000,000
Contingent convertible preferred stock, shares issued	1,099,877	1,099,877
Contingent convertible preferred stock, shares outstanding	439,951	1,099,877
Exchangeable shares of subsidiary, shares issued	257,156	257,156
Exchangeable shares of subsidiary, shares outstanding	32,804	32,804
Treasury stock, par value (usd per share)	$ 0.01	$ 0.01
Treasury stock, shares	10,092,852	9,024,679