Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Sustainable Growth Advisers, LP
Business Acquisition [Line Items]
Total Revenues	$ 152,210	$ 131,307	$ 431,400	$ 317,879
Net Income (Loss) Attributable to Common Stockholders	$ 26,201	$ 16,493	$ 69,284	$ 22,897
Basic EPS (in dollars per share)	$ 3.65	$ 2.29	$ 9.63	$ 3.30
Diluted EPS (in dollars per share)	$ 3.35	$ 2.19	$ 8.93	$ 3.19
RidgeWorth
Business Acquisition [Line Items]
Total Revenues		$ 118,010		$ 361,070
Net Income (Loss) Attributable to Common Stockholders		$ 13,696		$ 29,975
Basic EPS (in dollars per share)		$ 1.74		$ 3.42
Diluted EPS (in dollars per share)		$ 1.70		$ 3.31