Segments - Reconciling Adjustments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($) segment	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments (in segments) | segment	3	3	3	3
ENI revenue	$ 174.7	$ 207.1	$ 357.3	$ 414.3
ENI operating expenses	90.4	97.7	143.0	170.8
Earnings before variable compensation	84.3	109.4	214.3	243.5
Variable compensation	47.1	49.0	93.2	101.7
ENI operating earnings (after variable comp)	37.2	60.4	121.1	141.8
Affiliate key employee distributions	11.0	13.8	20.8	27.2
Earnings after Affiliate key employee distributions	26.2	46.6	100.3	114.6
Net interest expense	(7.2)	(8.5)	(14.7)	(14.4)
Net investment income	42.2	(2.4)	11.3	18.2
Net income (loss) attributable to non-controlling interests in consolidated Funds	(35.0)	6.4	(24.5)	(2.0)
Income tax (expense) benefit	(7.3)	(14.1)	(20.9)	(35.7)
Economic net income	18.9	28.0	51.5	80.7
Reconciling Adjustments
Segment Reporting Information [Line Items]
ENI revenue	2.2	2.5	4.2	4.0
ENI operating expenses	16.4	14.7	(12.2)	(0.7)
Earnings before variable compensation	(14.2)	(12.2)	16.4	4.7
Variable compensation	7.7	0.6	14.1	4.6
ENI operating earnings (after variable comp)	(21.9)	(12.8)	2.3	0.1
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(21.9)	(12.8)	2.3	0.1
Net interest expense	(1.6)	(2.3)	(3.5)	(4.7)
Net investment income	42.2	(2.4)	11.3	18.2
Net income (loss) attributable to non-controlling interests in consolidated Funds	(35.0)	6.4	(24.5)	(2.0)
Income tax (expense) benefit	2.3	(1.9)	(1.3)	(11.1)
Economic net income	(14.0)	(13.0)	(15.7)	0.5
Quant & Solutions
Segment Reporting Information [Line Items]
ENI revenue	82.8	93.7	168.9	184.2
ENI operating expenses	35.0	39.7	72.1	79.6
Earnings before variable compensation	47.8	54.0	96.8	104.6
Variable compensation	17.7	20.0	34.7	38.7
ENI operating earnings (after variable comp)	30.1	34.0	62.1	65.9
Affiliate key employee distributions	1.3	1.6	2.1	2.6
Earnings after Affiliate key employee distributions	28.8	32.4	60.0	63.3
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net income (loss) attributable to non-controlling interests in consolidated Funds	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Economic net income	28.8	32.4	60.0	63.3
Alter-natives
Segment Reporting Information [Line Items]
ENI revenue	42.5	44.3	84.4	88.3
ENI operating expenses	15.7	15.6	32.3	33.3
Earnings before variable compensation	26.8	28.7	52.1	55.0
Variable compensation	9.7	10.4	18.9	20.2
ENI operating earnings (after variable comp)	17.1	18.3	33.2	34.8
Affiliate key employee distributions	6.9	6.4	12.7	12.1
Earnings after Affiliate key employee distributions	10.2	11.9	20.5	22.7
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net income (loss) attributable to non-controlling interests in consolidated Funds	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Economic net income	10.2	11.9	20.5	22.7
Liquid Alpha
Segment Reporting Information [Line Items]
ENI revenue	47.1	66.5	99.6	137.6
ENI operating expenses	17.4	19.9	37.2	41.4
Earnings before variable compensation	29.7	46.6	62.4	96.2
Variable compensation	11.2	15.7	23.7	32.6
ENI operating earnings (after variable comp)	18.5	30.9	38.7	63.6
Affiliate key employee distributions	2.8	5.8	6.0	12.5
Earnings after Affiliate key employee distributions	15.7	25.1	32.7	51.1
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net income (loss) attributable to non-controlling interests in consolidated Funds	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Economic net income	15.7	25.1	32.7	51.1
Other
Segment Reporting Information [Line Items]
ENI revenue	0.1	0.1	0.2	0.2
ENI operating expenses	5.9	7.8	13.6	17.2
Earnings before variable compensation	(5.8)	(7.7)	(13.4)	(17.0)
Variable compensation	0.8	2.3	1.8	5.6
ENI operating earnings (after variable comp)	(6.6)	(10.0)	(15.2)	(22.6)
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(6.6)	(10.0)	(15.2)	(22.6)
Net interest expense	(5.6)	(6.2)	(11.2)	(9.7)
Net investment income	0.0	0.0	0.0	0.0
Net income (loss) attributable to non-controlling interests in consolidated Funds	0.0	0.0	0.0	0.0
Income tax (expense) benefit	(9.6)	(12.2)	(19.6)	(24.6)
Economic net income	$ (21.8)	$ (28.4)	$ (46.0)	$ (56.9)