Segments - Reconciling Adjustments (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($) segment	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments (in segments) | segment	3	3	3	3
ENI revenue	$ 182.4	$ 197.8	$ 539.7	$ 612.1
ENI operating expenses	86.4	89.2	229.4	260.0
Earnings before variable compensation	96.0	108.6	310.3	352.1
Variable compensation	44.5	44.1	137.7	145.8
ENI operating earnings (after variable comp)	51.5	64.5	172.6	206.3
Affiliate key employee distributions	11.0	12.6	31.8	39.8
Earnings after Affiliate key employee distributions	40.5	51.9	140.8	166.5
Net interest expense	(6.9)	(7.9)	(21.6)	(22.3)
Net investment income	6.0	7.0	17.3	25.2
Gain on sale of Affiliate	7.2		7.2
Net income attributable to non-controlling interests in consolidated Funds	3.2	(7.6)	(21.3)	(9.6)
Income tax (expense) benefit	(12.8)	32.0	(33.7)	(3.7)
Economic net income	37.2	75.4	88.7	156.1
Reconciling Adjustments
Segment Reporting Information [Line Items]
ENI revenue	1.6	2.0	5.8	6.0
ENI operating expenses	12.6	5.5	0.4	4.8
Earnings before variable compensation	(11.0)	(3.5)	5.4	1.2
Variable compensation	3.3	0.0	17.4	4.6
ENI operating earnings (after variable comp)	(14.3)	(3.5)	(12.0)	(3.4)
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(14.3)	(3.5)	(12.0)	(3.4)
Net interest expense	(1.6)	(2.2)	(5.1)	(6.9)
Net investment income	6.0	7.0	17.3	25.2
Gain on sale of Affiliate	7.2		7.2
Net income attributable to non-controlling interests in consolidated Funds	3.2	(7.6)	(21.3)	(9.6)
Income tax (expense) benefit	(1.0)	44.3	(2.3)	33.2
Economic net income	(0.5)	38.0	(16.2)	38.5
Quant & Solutions
Segment Reporting Information [Line Items]
ENI revenue	89.0	93.1	257.9	277.3
ENI operating expenses	37.2	40.8	109.3	120.4
Earnings before variable compensation	51.8	52.3	148.6	156.9
Variable compensation	18.0	18.8	52.7	57.6
ENI operating earnings (after variable comp)	33.8	33.5	95.9	99.3
Affiliate key employee distributions	0.8	1.5	2.9	4.1
Earnings after Affiliate key employee distributions	33.0	32.0	93.0	95.2
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Gain on sale of Affiliate	0.0		0.0
Net income 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	33.0	32.0	93.0	95.2
Alter-natives
Segment Reporting Information [Line Items]
ENI revenue	43.8	37.4	128.2	125.7
ENI operating expenses	15.3	16.1	47.6	49.4
Earnings before variable compensation	28.5	21.3	80.6	76.3
Variable compensation	10.1	7.8	29.0	28.0
ENI operating earnings (after variable comp)	18.4	13.5	51.6	48.3
Affiliate key employee distributions	7.3	5.0	20.0	17.1
Earnings after Affiliate key employee distributions	11.1	8.5	31.6	31.2
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Gain on sale of Affiliate	0.0		0.0
Net income 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	11.1	8.5	31.6	31.2
Liquid Alpha
Segment Reporting Information [Line Items]
ENI revenue	47.9	65.2	147.5	202.8
ENI operating expenses	15.5	18.5	52.7	60.0
Earnings before variable compensation	32.4	46.7	94.8	142.8
Variable compensation	12.3	15.6	36.0	48.1
ENI operating earnings (after variable comp)	20.1	31.1	58.8	94.7
Affiliate key employee distributions	2.9	6.1	8.9	18.6
Earnings after Affiliate key employee distributions	17.2	25.0	49.9	76.1
Net interest expense	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Gain on sale of Affiliate	0.0		0.0
Net income 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	17.2	25.0	49.9	76.1
Other
Segment Reporting Information [Line Items]
ENI revenue	0.1	0.1	0.3	0.3
ENI operating expenses	5.8	8.3	19.4	25.4
Earnings before variable compensation	(5.7)	(8.2)	(19.1)	(25.1)
Variable compensation	0.8	1.9	2.6	7.5
ENI operating earnings (after variable comp)	(6.5)	(10.1)	(21.7)	(32.6)
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(6.5)	(10.1)	(21.7)	(32.6)
Net interest expense	(5.3)	(5.7)	(16.5)	(15.4)
Net investment income	0.0	0.0	0.0	0.0
Gain on sale of Affiliate	0.0		0.0
Net income attributable to non-controlling interests in consolidated Funds	0.0	0.0	0.0	0.0
Income tax (expense) benefit	(11.8)	(12.3)	(31.4)	(36.9)
Economic net income	$ (23.6)	$ (28.1)	$ (69.6)	$ (84.9)