Goodwill and Other Intangible Assets - Indefinite-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Indefinite Lived Intangible Assets
Net carrying amount of unamortized indefinite lived intangible assets		$ 809.5	$ 797.8
WM Advisors, Inc.
Indefinite Lived Intangible Assets
Net carrying amount of unamortized indefinite lived intangible assets		$ 608.0	$ 608.0
Operating expense | Brazil mutual fund company
Indefinite Lived Intangible Assets
Indefinite lived intangible asset impairment	$ 14.7