Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
ENI revenue	$ 91.8	$ 112.2
ENI operating expenses	47.6	39.2
Earnings Before Variable Compensation	44.2	73.0
Variable compensation	23.2	27.9
Operating Earnings After Variable Compensation	21.0	45.1
Affiliate key employee distributions	1.2	1.9
Earnings after Affiliate key employee distributions	19.8	43.2
Net interest expense	(3.8)	(6.5)
Interest income	1.1	0.0
Investment income (loss)	0.3	(0.1)
Loss on extinguishment of debt	0.0	(3.2)
Income tax expense	(5.1)	(9.6)
Net income attributable to controlling interests	12.0	23.8
Operating Segments | Quant & Solutions
Segment Reporting Information [Line Items]
ENI revenue	91.1	112.2
ENI operating expenses	43.6	41.1
Earnings Before Variable Compensation	47.5	71.1
Variable compensation	22.5	26.3
Operating Earnings After Variable Compensation	25.0	44.8
Affiliate key employee distributions	1.2	1.9
Earnings after Affiliate key employee distributions	23.8	42.9
Net interest expense	0.0	0.0
Interest income	0.0
Investment income (loss)		0.0
Loss on extinguishment of debt		0.0
Income tax expense	0.0	0.0
Net income attributable to controlling interests	23.8	42.9
Other
Segment Reporting Information [Line Items]
ENI revenue	0.0	0.0
ENI operating expenses	3.5	4.5
Earnings Before Variable Compensation	(3.5)	(4.5)
Variable compensation	0.7	1.6
Operating Earnings After Variable Compensation	(4.2)	(6.1)
Affiliate key employee distributions	0.0	0.0
Earnings after Affiliate key employee distributions	(4.2)	(6.1)
Net interest expense	(3.4)	(4.6)
Interest income	0.0
Investment income (loss)		0.0
Loss on extinguishment of debt		0.0
Income tax expense	(4.4)	(8.8)
Net income attributable to controlling interests	(12.0)	(19.5)
Reconciling Adjustments
Segment Reporting Information [Line Items]
ENI revenue	0.7	0.0
ENI operating expenses	0.5	(6.4)
Earnings Before Variable Compensation	0.2	6.4
Variable compensation	0.0	0.0
Operating Earnings After Variable Compensation	0.2	6.4
Affiliate key employee distributions	0.0	0.0
Earnings after Affiliate key employee distributions	0.2	6.4
Net interest expense	(0.4)	(1.9)
Interest income	1.1
Investment income (loss)		(0.1)
Loss on extinguishment of debt		(3.2)
Income tax expense	(0.7)	(0.8)
Net income attributable to controlling interests	$ 0.2	$ 0.4