Business Combinations - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Mar. 31, 2020	May 24, 2019	Mar. 31, 2020	Mar. 31, 2019
Business Acquisition [Line Items]
Transaction, integration, and restructuring			$ 75.5	$ 46.1
OppenheimerFunds
Business Acquisition [Line Items]
Cash consideration	$ 35.0	$ 35.0
Preferred shares dividend rate (percent)			5.90%
Liability adjustment			$ 380.5
Deferred tax asset adjustment			93.5
Goodwill adjustment			287.0
Liquidation preference per share (in usd per share)		$ 1,000
Transaction, integration, and restructuring			71.1
Additional losses recognized for post-acquisition and related costs			$ 7.3
Common and Restricted Stock | OppenheimerFunds
Business Acquisition [Line Items]
Shares of common stock issued (shares)		81,900,000
Preferred shares | OppenheimerFunds
Business Acquisition [Line Items]
Perpetual, non-cumulative preferred shares issued	4,010.5	$ 4,000.0
Non-call period		21 years
Preferred shares dividend rate (percent)		5.90%
Lock-up period		5 years
Other equity securities | OppenheimerFunds
Business Acquisition [Line Items]
Shares of common stock issued (shares)		75,700,000
Perpetual, non-cumulative preferred shares issued	$ 1,453.6
Lock-up period		2 years
Restricted stock awards | OppenheimerFunds
Business Acquisition [Line Items]
Shares of common stock issued (shares)		6,200,000
MassMutual | Preferred shares | OppenheimerFunds
Business Acquisition [Line Items]
Approximate stake help in common stock of combined firm (percent)	16.50%		16.50%
Management contracts | OppenheimerFunds
Business Acquisition [Line Items]
Finite lived intangible assets acquired		$ 255.0
Estimated useful life		8 years
Trade names | OppenheimerFunds
Business Acquisition [Line Items]
Finite lived intangible assets acquired		$ 27.0
Estimated useful life		6 years
Management contracts | Management contracts | OppenheimerFunds
Business Acquisition [Line Items]
Indefinite-lived intangible assets acquired		$ 4,907.0
MassMutual | Other equity securities | OppenheimerFunds
Business Acquisition [Line Items]
Shares of common stock issued (shares)		75,563,041
Share price (in usd per share)		$ 19.195
Others | Other equity securities | OppenheimerFunds
Business Acquisition [Line Items]
Shares of common stock issued (shares)		153,574
Share price (in usd per share)		$ 20.42