Common Share-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common share-based compensation expense	$ 138.2	$ 158.8
Fair value of vested shares	$ 109.6	$ 161.0
Weighted average fair value of shares granted (usd per share)	$ 14.08	$ 14.47
Unrecognized compensation cost related to non-vested shares	$ 276.0
Weighted average non-vested shares compensation cost expected to recognize	2 years 4 months 28 days
Restricted stock awards \| OppenheimerFunds
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock issued (shares)		6.2

Number of shares of equity interests issued or issuable to acquire entity.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of noncash expense for share-based payment arrangement.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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