Segments (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments (in segments) | segment			3
ENI Revenue	$ 197.8	$ 230.1	$ 612.1	$ 713.7
ENI Operating Expenses	89.2	139.1	260.0	414.2
Earnings Before Variable Compensation	108.6	91.0	352.1	299.5
Variable compensation	44.1	57.2	145.8	182.4
ENI operating earnings (after variable comp)	64.5	33.8	206.3	117.1
Affiliate key employee distributions	12.6	20.5	39.8	62.9
Earnings after Affiliate key employee distributions	51.9	13.3	166.5	54.2
Net interest income (expense)	(7.9)	(5.4)	(22.3)	(16.7)
Net consolidated Funds’ investment gains (losses)	7.0	2.8	25.2	62.8
Net income attributable to non-controlling interests in consolidated Funds	(7.6)	(0.2)	(9.6)	1.9
Income tax expense (benefit)	32.0	43.4	(3.7)	11.1
Gain (loss) on disposal of discontinued operations, net of tax	0.0	0.1	0.0	0.1
Net income	83.0	54.2	165.7	111.5
Net Income (Loss) Attributable to Parent	75.4	54.0	156.1	113.4
Quant & Solutions
Segment Reporting Information [Line Items]
ENI Revenue	93.1	97.0	277.3	295.2
ENI Operating Expenses	40.8	37.5	120.4	107.2
Earnings Before Variable Compensation	52.3	59.5	156.9	188.0
Variable compensation	18.8	21.1	57.6	67.0
ENI operating earnings (after variable comp)	33.5	38.4	99.3	121.0
Affiliate key employee distributions	1.5	2.4	4.1	8.4
Earnings after Affiliate key employee distributions	32.0	36.0	95.2	112.6
Net income	32.0	36.0	95.2	112.6
Alter-natives
Segment Reporting Information [Line Items]
ENI Revenue	37.4	54.6	125.7	170.0
ENI Operating Expenses	16.1	15.5	49.4	46.7
Earnings Before Variable Compensation	21.3	39.1	76.3	123.3
Variable compensation	7.8	14.3	28.0	45.7
ENI operating earnings (after variable comp)	13.5	24.8	48.3	77.6
Affiliate key employee distributions	5.0	10.1	17.1	28.5
Earnings after Affiliate key employee distributions	8.5	14.7	31.2	49.1
Net income	8.5	14.7	31.2	49.1
Liquid Alpha
Segment Reporting Information [Line Items]
ENI Revenue	65.2	76.9	202.8	241.6
ENI Operating Expenses	18.5	21.0	60.0	64.1
Earnings Before Variable Compensation	46.7	55.9	142.8	177.5
Variable compensation	15.6	18.6	48.1	58.2
ENI operating earnings (after variable comp)	31.1	37.3	94.7	119.3
Affiliate key employee distributions	6.1	8.0	18.6	26.0
Earnings after Affiliate key employee distributions	25.0	29.3	76.1	93.3
Net income	25.0	29.3	76.1	93.3
Other
Segment Reporting Information [Line Items]
ENI Revenue	0.1	0.1	0.3	0.3
ENI Operating Expenses	8.3	9.6	25.4	31.6
Earnings Before Variable Compensation	(8.2)	(9.5)	(25.1)	(31.3)
Variable compensation	1.9	3.2	7.5	11.5
ENI operating earnings (after variable comp)	(10.1)	(12.7)	(32.6)	(42.8)
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(10.1)	(12.7)	(32.6)	(42.8)
Net interest income (expense)	(5.7)	(3.1)	(15.4)	(10.8)
Income tax expense (benefit)	(12.3)	(15.4)	(36.9)	(47.2)
Net income	(28.1)	(31.2)	(84.9)	(100.8)
Reconciling Adjustments
Segment Reporting Information [Line Items]
ENI Revenue	2.0	1.5	6.0	6.6
ENI Operating Expenses	5.5	55.5	4.8	164.6
Earnings Before Variable Compensation	(3.5)	(54.0)	1.2	(158.0)
Variable compensation	0.0	0.0	4.6	0.0
ENI operating earnings (after variable comp)	(3.5)	(54.0)	(3.4)	(158.0)
Affiliate key employee distributions	0.0	0.0	0.0	0.0
Earnings after Affiliate key employee distributions	(3.5)	(54.0)	(3.4)	(158.0)
Net interest income (expense)	(2.2)	(2.3)	(6.9)	(5.9)
Net consolidated Funds’ investment gains (losses)	7.0	2.8	25.2	62.8
Net income attributable to non-controlling interests in consolidated Funds	(7.6)	(0.2)	(9.6)	1.9
Income tax expense (benefit)	44.3	58.8	33.2	58.3
Gain (loss) on disposal of discontinued operations, net of tax		0.1		0.1
Net income	$ 38.0	$ 5.2	$ 38.5	$ (40.8)