Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Cash and cash equivalents, restricted from use	$ 3.8	$ 3.9
Advisory fees receivable, allowance for doubtful accounts	0.0	0.3
Property and equipment, accumulated depreciation	$ 43.9	$ 62.1
Common stock, par value (usd per share)	$ 0.01	$ 0.01
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	44,823,361	43,750,170
Common stock, shares outstanding (shares)	22,873,749	27,084,520
Exchangeable shares of subsidiary, shares issued (shares)	257,156	257,156
Exchangeable shares of subsidiary, shares outstanding (shares)	32,804	32,804
Treasury stock, par value (usd per share)	$ 0.01	$ 0.01
Treasury stock, shares (shares)	21,949,612	16,665,560