Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Asset of BSIF and consolidated Funds
Investments at fair value	$ 332.2	$ 321.4
Total fair value assets	332.2	321.4
Liabilities of consolidated Funds
Total fair value liabilities	(1.6)	(1.7)
Investment funds
Asset of BSIF and consolidated Funds
Uncategorized	$ 54.8	97.4
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
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	$ 14.3	13.8
Short-term investment funds	2.9	2.2
Bank loans	104.8	63.9
Other investments	5.3	42.5
Derivatives	0.8	2.4
Investments at fair value	128.1	124.8
Total fair value assets	128.1	124.8
Liabilities of consolidated Funds
Common stock		(0.8)
Derivatives	(1.6)	(0.9)
Total fair value liabilities	(1.6)	(1.7)
Consolidated Funds \| Other Investments
Asset of BSIF and consolidated Funds
Uncategorized	0.0	38.8
Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	46.5	43.2
Investments related to long-term incentive compensation plans	99.8	91.8
Uncategorized	54.8
Total fair value assets	204.1	196.6
Investments in unconsolidated Funds \| Investment funds
Asset of BSIF and consolidated Funds
Uncategorized	54.8	58.6
Investments at fair value	57.8	61.6
Investments in unconsolidated Funds \| Investment Funds Subject To Longer Redemption Restrictions
Asset of BSIF and consolidated Funds
Uncategorized	9.3	12.3
Quoted prices in active markets (Level I)
Asset of BSIF and consolidated Funds
Total fair value assets	157.9	148.7
Liabilities of consolidated Funds
Total fair value liabilities	(1.4)	(1.6)
Quoted prices in active markets (Level I) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	14.3	13.8
Short-term investment funds	2.9	2.2
Bank loans	0.0	0.0
Other investments	4.3	3.7
Derivatives	0.7	2.2
Total fair value assets	22.2	21.9
Liabilities of consolidated Funds
Common stock		(0.8)
Derivatives	(1.4)	(0.8)
Total fair value liabilities	(1.4)	(1.6)
Quoted prices in active markets (Level I) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	35.9	35.0
Investments related to long-term incentive compensation plans	99.8	91.8
Total fair value assets	$ 135.7	$ 126.8
Liabilities of consolidated Funds
Investment in cash	8.00%	11.00%
Investment in equity securities	92.00%	89.00%
Quoted prices in active markets (Level I) \| Investments in unconsolidated Funds \| Investment funds
Asset of BSIF and consolidated Funds
Investments at fair value	$ 0.0	$ 0.0
Significant other observable inputs (Level II)
Asset of BSIF and consolidated Funds
Total fair value assets	116.5	72.3
Liabilities of consolidated Funds
Total fair value liabilities	(0.2)	(0.1)
Significant other observable inputs (Level II) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	0.0	0.0
Short-term investment funds	0.0	0.0
Bank loans	104.8	63.9
Other investments	1.0	0.0
Derivatives	0.1	0.2
Total fair value assets	105.9	64.1
Liabilities of consolidated Funds
Common stock		0.0
Derivatives	(0.2)	(0.1)
Total fair value liabilities	(0.2)	(0.1)
Significant other observable inputs (Level II) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	10.6	8.2
Investments related to long-term incentive compensation plans	0.0	0.0
Total fair value assets	10.6	8.2
Significant other observable inputs (Level II) \| Investments in unconsolidated Funds \| Investment funds
Asset of BSIF and consolidated Funds
Investments at fair value	0.0	0.0
Significant unobservable inputs (Level III)
Asset of BSIF and consolidated Funds
Total fair value assets	3.0	3.0
Liabilities of consolidated Funds
Total fair value liabilities	0.0	0.0
Significant unobservable inputs (Level III) \| Consolidated Funds
Asset of BSIF and consolidated Funds
Common and preferred stock	0.0	0.0
Short-term investment funds	0.0	0.0
Bank loans	0.0	0.0
Other investments	0.0	0.0
Derivatives	0.0	0.0
Total fair value assets	0.0	0.0
Liabilities of consolidated Funds
Common stock		0.0
Derivatives	0.0	0.0
Total fair value liabilities	0.0	0.0
Significant unobservable inputs (Level III) \| Investments in unconsolidated Funds
Asset of BSIF and consolidated Funds
Investments in separate accounts	0.0	0.0
Investments related to long-term incentive compensation plans	0.0	0.0
Total fair value assets	3.0	3.0
Significant unobservable inputs (Level III) \| Investments in unconsolidated Funds \| Investment funds
Asset of BSIF and consolidated Funds
Investments at fair value	$ 3.0	$ 3.0

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