Fair Value Measurements (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Asset of BSIF and consolidated Funds
Investments at fair value	$ 266,200,000		$ 303,800,000
Total fair value assets	266,200,000		303,800,000
Liabilities of consolidated Funds
Total fair value liabilities	$ (3,100,000)		(900,000)
Various Investments
Asset of BSIF and consolidated Funds
Uncategorized			48,200,000
Real estate investment funds \| Minimum
Asset of BSIF and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	1 year
Real estate investment funds \| Maximum
Asset of BSIF and consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	11 years
Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	$ 7,200,000		9,800,000
Short-term investment funds	400,000		100,000
Bank loans	93,900,000		109,000,000.0
Derivatives	6,100,000		600,000
Investments at fair value	107,600,000		119,500,000
Total fair value assets	107,600,000		119,500,000
Liabilities of consolidated Funds
Common stock			(500,000)
Derivatives	(3,100,000)		(400,000)
Total fair value liabilities	(3,100,000)		(900,000)
Consolidated Funds \| Other Investments
Asset of BSIF and consolidated Funds
Uncategorized	0		0
Investments in unconsolidated Funds
Fair value measurements
Additions and transfers into Level III	0	$ 0
Asset of BSIF and consolidated Funds
Investments in separate accounts	37,400,000		44,300,000
Investments related to long-term incentive compensation plans	89,300,000		88,800,000
Uncategorized	28,900,000
Total fair value assets	158,600,000		184,300,000
Investments in unconsolidated Funds \| Various Investments
Asset of BSIF and consolidated Funds
Uncategorized	28,900,000		48,200,000
Investments at fair value	31,900,000		51,200,000
Investments in unconsolidated Funds \| Investment Funds Subject To Longer Redemption Restrictions
Asset of BSIF and consolidated Funds
Uncategorized	5,900,000		6,400,000
Consolidated Entity Funds
Fair value measurements
Additions and transfers into Level III	0
Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Total fair value assets	130,200,000		132,400,000
Liabilities of consolidated Funds
Total fair value liabilities	(2,600,000)		(600,000)
Quoted prices in active markets (Level I) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	7,200,000		9,800,000
Short-term investment funds	400,000		100,000
Bank loans	0		0
Derivatives	5,400,000		500,000
Total fair value assets	13,000,000.0		10,400,000
Liabilities of consolidated Funds
Common stock			(500,000)
Derivatives	(2,600,000)		(100,000)
Total fair value liabilities	(2,600,000)		(600,000)
Quoted prices in active markets (Level I) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	27,900,000		33,200,000
Investments related to long-term incentive compensation plans	89,300,000		88,800,000
Total fair value assets	$ 117,200,000		$ 122,000,000.0
Liabilities of consolidated Funds
Investment in cash	30.00%		3.00%
Investment in equity securities	70.00%		97.00%
Quoted prices in active markets (Level I) \| Investments in unconsolidated Funds \| Various Investments
Asset of BSIF and consolidated Funds
Investments at fair value	$ 0		$ 0
Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Total fair value assets	104,100,000		120,200,000
Liabilities of consolidated Funds
Total fair value liabilities	(500,000)		(300,000)
Significant other observable inputs (Level II) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	0		0
Short-term investment funds	0		0
Bank loans	93,900,000		109,000,000.0
Derivatives	700,000		100,000
Total fair value assets	94,600,000		109,100,000
Liabilities of consolidated Funds
Common stock			0
Derivatives	(500,000)		(300,000)
Total fair value liabilities	(500,000)		(300,000)
Significant other observable inputs (Level II) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	9,500,000		11,100,000
Investments related to long-term incentive compensation plans	0		0
Total fair value assets	9,500,000		11,100,000
Significant other observable inputs (Level II) \| Investments in unconsolidated Funds \| Various Investments
Asset of BSIF and consolidated Funds
Investments at fair value	0		0
Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Total fair value assets	3,000,000.0		3,000,000.0
Liabilities of consolidated Funds
Total fair value liabilities	0		0
Significant unobservable inputs (Level III) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	0		0
Short-term investment funds	0		0
Bank loans	0		0
Derivatives	0		0
Total fair value assets	0		0
Liabilities of consolidated Funds
Common stock			0
Derivatives	0		0
Total fair value liabilities	0		0
Significant unobservable inputs (Level III) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	0		0
Investments related to long-term incentive compensation plans	0		0
Total fair value assets	3,000,000.0		3,000,000.0
Significant unobservable inputs (Level III) \| Investments in unconsolidated Funds \| Various Investments
Asset of BSIF and consolidated Funds
Investments at fair value	$ 3,000,000.0		$ 3,000,000.0

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