Segments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments (in segments) | segment	3
ENI Revenue	$ 182.6	$ 207.2
ENI Operating Expenses	52.6	73.1
Earnings Before Variable Compensation	130.0	134.1
Variable compensation	46.1	52.7
ENI operating earnings (after variable comp)	83.9	81.4
Affiliate key employee distributions	9.8	13.4
Earnings after Affiliate key employee distributions	74.1	68.0
Net interest income (expense)	(7.5)	(5.9)
Net consolidated Funds’ investment gains (losses)	(30.9)	20.6
Net income attributable to non-controlling interests in consolidated Funds	10.5	(8.4)
Income tax expense (benefit)	(13.6)	(21.6)
Gain (loss) on disposal of discontinued operations, net of tax	0.0	0.0
Net income	22.1	61.1
Net Income (Loss) Attributable to Parent	32.6	52.7
Quant & Solutions
Segment Reporting Information [Line Items]
ENI Revenue	86.1	90.5
ENI Operating Expenses	37.1	39.9
Earnings Before Variable Compensation	49.0	50.6
Variable compensation	17.0	18.8
ENI operating earnings (after variable comp)	32.0	31.8
Affiliate key employee distributions	0.8	1.0
Earnings after Affiliate key employee distributions	31.2	30.8
Net income	31.2	30.8
Alter-natives
Segment Reporting Information [Line Items]
ENI Revenue	41.9	44.0
ENI Operating Expenses	16.6	17.7
Earnings Before Variable Compensation	25.3	26.3
Variable compensation	9.2	9.8
ENI operating earnings (after variable comp)	16.1	16.5
Affiliate key employee distributions	5.8	5.7
Earnings after Affiliate key employee distributions	10.3	10.8
Net income	10.3	10.8
Liquid Alpha
Segment Reporting Information [Line Items]
ENI Revenue	52.5	71.1
ENI Operating Expenses	19.8	21.6
Earnings Before Variable Compensation	32.7	49.5
Variable compensation	12.5	16.8
ENI operating earnings (after variable comp)	20.2	32.7
Affiliate key employee distributions	3.2	6.7
Earnings after Affiliate key employee distributions	17.0	26.0
Net income	17.0	26.0
Other
Segment Reporting Information [Line Items]
ENI Revenue	0.1	0.1
ENI Operating Expenses	7.7	9.3
Earnings Before Variable Compensation	(7.6)	(9.2)
Variable compensation	1.0	3.3
ENI operating earnings (after variable comp)	(8.6)	(12.5)
Affiliate key employee distributions	0.0	0.0
Earnings after Affiliate key employee distributions	(8.6)	(12.5)
Net interest income (expense)	(5.6)	(3.5)
Income tax expense (benefit)	10.0	12.4
Net income	(24.2)	(28.4)
Reconciling Adjustments
Segment Reporting Information [Line Items]
ENI Revenue	2.0	1.5
ENI Operating Expenses	(28.6)	(15.4)
Earnings Before Variable Compensation	30.6	16.9
Variable compensation	6.4	4.0
ENI operating earnings (after variable comp)	24.2	12.9
Affiliate key employee distributions	0.0	0.0
Earnings after Affiliate key employee distributions	24.2	12.9
Net interest income (expense)	(1.9)	(2.4)
Net consolidated Funds’ investment gains (losses)	(30.9)	20.6
Net income attributable to non-controlling interests in consolidated Funds	10.5	(8.4)
Income tax expense (benefit)	(3.6)	(9.2)
Net income	$ (1.7)	$ 13.5