Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Assets of BSIG and consolidated Funds
Investments in unconsolidated Funds	$ 54.5	$ 88.0
Total fair value assets	54.5	88.0
Real Estate Funds
Assets of BSIG and consolidated Funds
Uncategorized	$ 4.9	16.1
Minimum
Assets of BSIG and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	1 year
Maximum
Assets of BSIG and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	2 years
Consolidated Entity Excluding Consolidated Funds
Assets of BSIG and consolidated Funds
Investments in separate accounts	$ 4.6	21.3
Investments related to long-term incentive compensation plans	45.0	48.0
Uncategorized	4.8	6.2
Investments in unconsolidated Funds	54.5	88.0
Consolidated Entity Excluding Consolidated Funds \| Real Estate Funds
Assets of BSIG and consolidated Funds
Uncategorized	4.9	16.1
Investments in unconsolidated Funds	4.9	18.7
Consolidated Entity Excluding Consolidated Funds \| Uncategorized
Assets of BSIG and consolidated Funds
Uncategorized	4.9	16.1
Quoted prices in active markets (Level I)
Assets of BSIG and consolidated Funds
Total fair value assets	49.6	57.7
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds
Assets of BSIG and consolidated Funds
Investments in separate accounts	4.6	9.7
Investments related to long-term incentive compensation plans	$ 45.0	$ 48.0
Investment, fair value disclosure, percentage of investment held in cash		11.00%
Investment, fair value disclosure, percentage of investment held in equity securities	100.00%	89.00%
Quoted prices in active markets (Level I) \| Consolidated Entity Excluding Consolidated Funds \| Real Estate Funds
Assets of BSIG and consolidated Funds
Investments in unconsolidated Funds	$ 0.0	$ 0.0
Significant other observable inputs (Level II)
Assets of BSIG and consolidated Funds
Total fair value assets	0.0	11.6
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds
Assets of BSIG and consolidated Funds
Investments in separate accounts	0.0	11.6
Investments related to long-term incentive compensation plans	0.0	0.0
Significant other observable inputs (Level II) \| Consolidated Entity Excluding Consolidated Funds \| Real Estate Funds
Assets of BSIG and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Significant unobservable inputs (Level III)
Assets of BSIG and consolidated Funds
Total fair value assets	0.0	2.6
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds
Assets of BSIG and consolidated Funds
Investments in separate accounts	0.0	0.0
Investments related to long-term incentive compensation plans	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Entity Excluding Consolidated Funds \| Real Estate Funds
Assets of BSIG and consolidated Funds
Investments in unconsolidated Funds	$ 0.0	$ 2.6

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

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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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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 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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