Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025	Dec. 31, 2024
Asset of BSIF and consolidated Funds
Total fair value assets	$ 225.7	$ 221.9
Uncategorized	19.3	19.4
Liabilities of consolidated Funds
Total fair value liabilities	$ (26.0)	(20.7)
Real estate investment funds
Liabilities of consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	1 year
Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Total fair value assets	$ 143.1	142.7
Liabilities of consolidated Funds
Total fair value liabilities	(25.7)	(20.4)
Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Total fair value assets	63.3	59.8
Liabilities of consolidated Funds
Total fair value liabilities	(0.3)	(0.3)
Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Total fair value assets	0.0	0.0
Liabilities of consolidated Funds
Total fair value liabilities	0.0	0.0
Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	107.1	94.2
Derivatives	0.8	0.7
Investments in unconsolidated Funds	170.5	154.0
Liabilities of consolidated Funds
Securities sold short	(25.7)	(20.4)
Derivatives	(0.3)	(0.3)
Consolidated Funds | Corporate bonds
Asset of BSIF and consolidated Funds
Corporate bonds	62.6	59.1
Consolidated Funds | Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Common and preferred stock	107.1	94.2
Derivatives	0.1	0.0
Liabilities of consolidated Funds
Securities sold short	(25.7)	(20.4)
Derivatives	0.0	0.0
Consolidated Funds | Quoted prices in active markets (Level I) | Corporate bonds
Asset of BSIF and consolidated Funds
Corporate bonds	0.0	0.0
Consolidated Funds | Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Common and preferred stock	0.0	0.0
Derivatives	0.7	0.7
Liabilities of consolidated Funds
Securities sold short	0.0	0.0
Derivatives	(0.3)	(0.3)
Consolidated Funds | Significant other observable inputs (Level II) | Corporate bonds
Asset of BSIF and consolidated Funds
Corporate bonds	62.6	59.1
Consolidated Funds | Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Common and preferred stock	0.0	0.0
Derivatives	0.0	0.0
Liabilities of consolidated Funds
Securities sold short	0.0	0.0
Derivatives	0.0	0.0
Consolidated Funds | Significant unobservable inputs (Level III) | Corporate bonds
Asset of BSIF and consolidated Funds
Corporate bonds	0.0	0.0
Consolidated Entity Excluding Consolidated Funds
Asset of BSIF and consolidated Funds
Consolidated Funds total	170.5	154.0
AAMI total	55.2	67.9
Uncategorized	19.3	19.4
Liabilities of consolidated Funds
Consolidated Funds total	(26.0)	(20.7)
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	35.9	48.5
Investments in unconsolidated Funds	35.9	48.5
Consolidated Entity Excluding Consolidated Funds | Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	19.3	19.4
Uncategorized	19.3	19.4
Consolidated Entity Excluding Consolidated Funds | Investment funds subject to longer redemption restrictions
Asset of BSIF and consolidated Funds
Uncategorized	3.3	2.9
Consolidated Entity Excluding Consolidated Funds | Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Consolidated Funds total	107.2	94.2
AAMI total	35.9	48.5
Liabilities of consolidated Funds
Consolidated Funds total	(25.7)	(20.4)
Consolidated Entity Excluding Consolidated Funds | Quoted prices in active markets (Level I) | Investments related to long-term incentive compensation plans
Asset of BSIF and consolidated Funds
Investments related to long-term incentive compensation plans	35.9	48.5
Consolidated Entity Excluding Consolidated Funds | Quoted prices in active markets (Level I) | Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Consolidated Funds total	63.3	59.8
AAMI total	0.0	0.0
Liabilities of consolidated Funds
Consolidated Funds total	(0.3)	(0.3)
Consolidated Entity Excluding Consolidated Funds | Significant other observable inputs (Level II) | 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
Consolidated Entity Excluding Consolidated Funds | Significant other observable inputs (Level II) | Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Consolidated Funds total	0.0	0.0
AAMI total	0.0	0.0
Liabilities of consolidated Funds
Consolidated Funds total	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant unobservable inputs (Level III) | 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
Consolidated Entity Excluding Consolidated Funds | Significant unobservable inputs (Level III) | Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in unconsolidated Funds	$ 0.0	$ 0.0