Goodwill and Other Intangible Assets - Indefinite-Lived Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Indefinite Lived Intangible Assets
Net carrying amount of unamortized indefinite lived intangible assets	$ 793.3	$ 809.5
WM Advisors, Inc.
Indefinite Lived Intangible Assets
Net carrying amount of unamortized indefinite lived intangible assets	$ 608.0	$ 608.0

Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Name:	us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant