Segment Information - Schedule of Economic Net Income by Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment information
ENI revenue	$ 207.4	$ 197.8	$ 207.1	$ 207.2	$ 214.5	$ 230.1	$ 233.9	$ 249.7	$ 819.5	$ 928.2	$ 887.4
ENI operating expenses									324.7	531.9	491.1
Earnings before variable compensation									494.8	396.3	396.3
Variable compensation									199.4	235.9	252.2
ENI operating earnings (after variable comp)									295.4	160.4	144.1
Affiliate key employee distributions									45.1	76.6	73.1
Operating Income (Loss)	83.8	51.9	46.6	68.0	29.6	13.3	15.3	25.6	250.3	83.8	71.0
Net interest income (expense)									(30.0)	(21.7)	(23.7)
Net investment income									37.7	53.1	42.9
Net income attributable to non-controlling interests in consolidated Funds									(16.1)	6.1	(4.9)
Income tax expense									(18.0)	(5.0)	(132.8)
Gain (loss) on disposal of discontinued operations, net of tax									0.0	0.1	(0.1)
Net Income (Loss) Attributable to Parent	$ 74.3	$ 83.0	$ 21.6	$ 61.1	$ 18.8	$ 54.2	$ 2.5	$ 54.8	223.9	136.4	4.2
Consolidated Entity Excluding Consolidated Funds
Segment information
Revaluation of DTA deed									0.0	20.0	51.8
Gain (loss) on disposal of discontinued operations, net of tax									0.0	0.1	(0.1)
Quant & Solutions
Segment information
ENI revenue									380.6	389.0	368.5
ENI operating expenses									160.6	146.3	126.6
Earnings before variable compensation									220.0	242.7	241.9
Variable compensation									75.6	86.2	79.2
ENI operating earnings (after variable comp)									144.4	156.5	162.7
Affiliate key employee distributions									6.4	9.5	8.6
Earnings after Affiliate key employee distributions									138.0	147.0	154.1
Net interest income (expense)									0.0
Net investment income									0.0
Net income attributable to non-controlling interests in consolidated Funds									0.0
Income tax expense									0.0
Economic Net Income									138.0	147.0	154.1
Alternatives
Segment information
ENI revenue									166.5	218.1	186.1
ENI operating expenses									66.9	61.8	57.4
Earnings before variable compensation									99.6	156.3	128.7
Variable compensation									36.7	58.9	48.7
ENI operating earnings (after variable comp)									62.9	97.4	80.0
Affiliate key employee distributions									23.0	34.1	26.8
Earnings after Affiliate key employee distributions									39.9	63.3	53.2
Net interest income (expense)									0.0
Net investment income									0.0
Net income attributable to non-controlling interests in consolidated Funds									0.0
Income tax expense									0.0
Economic Net Income									39.9	63.3	53.2
Liquid Alpha
Segment information
ENI revenue									263.8	311.6	335.1
ENI operating expenses									78.8	84.5	82.1
Earnings before variable compensation									185.0	227.1	253.0
Variable compensation									62.4	73.9	86.9
ENI operating earnings (after variable comp)									122.6	153.2	166.1
Affiliate key employee distributions									23.7	33.0	37.7
Earnings after Affiliate key employee distributions									98.9	120.2	128.4
Net interest income (expense)									0.0
Net investment income									0.0
Net income attributable to non-controlling interests in consolidated Funds									0.0
Income tax expense									0.0
Economic Net Income									98.9	120.2	128.4
Other
Segment information
ENI revenue									0.4	0.4	11.0
ENI operating expenses									35.4	43.1	48.0
Earnings before variable compensation									(35.0)	(42.7)	(37.0)
Variable compensation									10.0	11.7	28.6
ENI operating earnings (after variable comp)									(45.0)	(54.4)	(65.6)
Affiliate key employee distributions									0.0
Earnings after Affiliate key employee distributions									(45.0)	(54.4)	(65.6)
Net interest income (expense)									(21.0)	(13.6)	(18.8)
Net investment income									0.0
Net income attributable to non-controlling interests in consolidated Funds									0.0
Income tax expense									(50.0)	(62.7)	(70.4)
Economic Net Income									(116.0)	(130.7)	(154.8)
Reconciling Adjustments
Segment information
ENI revenue									8.2	9.1	(13.3)
ENI operating expenses									(17.0)	196.2	177.0
Earnings before variable compensation									25.2	(187.1)	(190.3)
Variable compensation									14.7	5.2	8.8
ENI operating earnings (after variable comp)									10.5	(192.3)	(199.1)
Affiliate key employee distributions									(8.0)
Earnings after Affiliate key employee distributions									18.5	(192.3)	(199.1)
Net interest income (expense)									(9.0)	(8.1)	(4.9)
Net investment income									37.7	53.1	42.9
Revaluation of DTA deed										20.0	51.8
Net income attributable to non-controlling interests in consolidated Funds									(16.1)	6.1	(4.9)
Income tax expense									32.0	57.7	(62.4)
Gain (loss) on disposal of discontinued operations, net of tax										0.1	(0.1)
Economic Net Income									$ 63.1	$ (63.4)	$ (176.7)