Fair Value Measurements - Summary of the Assets and Liabilities that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2023	Dec. 31, 2022
Asset of BSIF and consolidated Funds
Total fair value assets	$ 52.7	$ 50.3
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Derivative liabilities at fair value	Derivative liabilities at fair value
Total fair value liabilities	$ (2.6)	$ (2.2)
Investments in unconsolidated funds
Asset of BSIF and consolidated Funds
Uncategorized	$ 4.3	4.2
Minimum \| Real estate investment funds
Asset of BSIF and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	1 year
Maximum \| Real estate investment funds
Asset of BSIF and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	2 years
Consolidated Entity Excluding Consolidated Funds
Asset of BSIF and consolidated Funds
Derivatives	$ 2.8	1.9
Derivative asset Consolidated Funds total	2.8	1.9
Consolidated Funds total	49.9	48.4
Derivative Liability	(2.6)	(2.2)
Consolidated Funds total	(2.6)	(2.2)
Consolidated Entity Excluding Consolidated Funds \| Investments in separate accounts
Asset of BSIF and consolidated Funds
Investments in separate accounts	4.4	4.2
Consolidated Entity Excluding Consolidated Funds \| Investments related to long-term incentive compensation plans
Asset of BSIF and consolidated Funds
Investments related to long-term incentive compensation plans	41.2	40.0
Investments in unconsolidated Funds	41.2	40.0
Consolidated Entity Excluding Consolidated Funds \| Investments in unconsolidated funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	4.3	4.2
Uncategorized	4.3	4.2
Consolidated Entity Excluding Consolidated Funds \| Investment Funds Subject to Longer Redemption Restrictions
Asset of BSIF and consolidated Funds
Uncategorized	4.2	4.1
Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Total fair value assets	45.9	44.5
Total fair value liabilities	(0.3)	(0.2)
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds
Asset of BSIF and consolidated Funds
Derivatives	0.3	0.3
Derivative asset Consolidated Funds total	0.3	0.3
Consolidated Funds total	45.6	44.2
Uncategorized	4.3	4.2
Derivative Liability	(0.3)	(0.2)
Consolidated Funds total	$ (0.3)	$ (0.2)
Investment in equity securities	100.00%	100.00%
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds \| Investments in separate accounts
Asset of BSIF and consolidated Funds
Investments in separate accounts	$ 4.4	$ 4.2
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds \| Investments related to long-term incentive compensation plans
Asset of BSIF and consolidated Funds
Investments related to long-term incentive compensation plans	41.2	40.0
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds \| Investments in unconsolidated funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Total fair value assets	2.5	1.6
Total fair value liabilities	(2.3)	(2.0)
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds
Asset of BSIF and consolidated Funds
Derivatives	2.5	1.6
Derivative asset Consolidated Funds total	2.5	1.6
Consolidated Funds total	0.0	0.0
Derivative Liability	(2.3)	(2.0)
Consolidated Funds total	(2.3)	(2.0)
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds \| Investments in separate accounts
Asset of BSIF and consolidated Funds
Investments in separate accounts	0.0	0.0
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds \| Investments related to long-term incentive compensation plans
Asset of BSIF and consolidated Funds
Investments related to long-term incentive compensation plans	0.0	0.0
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds \| Investments in unconsolidated funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Total fair value assets	0.0	0.0
Total fair value liabilities	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds
Asset of BSIF and consolidated Funds
Derivatives	0.0	0.0
Derivative asset Consolidated Funds total	0.0	0.0
Consolidated Funds total	0.0	0.0
Derivative Liability	0.0	0.0
Consolidated Funds total	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds \| Investments in separate accounts
Asset of BSIF and consolidated Funds
Investments in separate accounts	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds \| Investments related to long-term incentive compensation plans
Asset of BSIF and consolidated Funds
Investments related to long-term incentive compensation plans	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds \| Investments in unconsolidated funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	$ 0.0	$ 0.0

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Derivative liabilities Consolidated Funds total

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Investment, Fair Value Disclosure, Percentage of Investment Held in Equity Securities

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Investments in Affiliates, Subsidiaries, Associates, and Joint Ventures Uncategorized, Fair Value Disclosure

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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Remaining period of expected liquidation by investee of underlying asset of investment measured at net asset value per share or unit. In 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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