Goodwill and Intangible Assets - Changes in Goodwill (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill gross, beginning balance			$ 308,500,000	$ 308,500,000	$ 308,500,000
Accumulated impairment, beginning balance			(33,900,000)	(33,900,000)	(33,900,000)
Goodwill, beginning balance			274,600,000	274,600,000	274,600,000
Additions			0	0
Impairments	$ 0	$ 0	(16,400,000)	0	0
Disposals			0	0
Decrease from reclassification to assets held for sale			(76,100,000)
Goodwill, ending balance	182,100,000	274,600,000	182,100,000	274,600,000	274,600,000
Accumulated impairment, ending balance	(50,300,000)	(33,900,000)	(50,300,000)	(33,900,000)	(33,900,000)
Goodwill gross, ending balance	232,400,000	308,500,000	232,400,000	308,500,000	308,500,000
Consolidated Entity Excluding Consolidated Funds
Goodwill [Roll Forward]
Goodwill, beginning balance			274,600,000
Impairments			(16,400,000)	0
Goodwill, ending balance	182,100,000		182,100,000		274,600,000
Disposal Group, Held-for-sale, Not Discontinued Operations | Barrow Hanley | Consolidated Entity Excluding Consolidated Funds
Goodwill [Roll Forward]
Goodwill	76,100,000		76,100,000
Quant & Solutions
Goodwill [Roll Forward]
Goodwill gross, beginning balance			22,100,000	22,100,000	22,100,000
Accumulated impairment, beginning balance			(1,800,000)	(1,800,000)	(1,800,000)
Goodwill, beginning balance			20,300,000	20,300,000	20,300,000
Additions			0	0
Impairments			0	0
Disposals			0	0
Decrease from reclassification to assets held for sale			0
Goodwill, ending balance	20,300,000	20,300,000	20,300,000	20,300,000	20,300,000
Accumulated impairment, ending balance	(1,800,000)	(1,800,000)	(1,800,000)	(1,800,000)	(1,800,000)
Goodwill gross, ending balance	22,100,000	22,100,000	22,100,000	22,100,000	22,100,000
Alternatives
Goodwill [Roll Forward]
Goodwill gross, beginning balance			153,100,000	153,100,000	153,100,000
Accumulated impairment, beginning balance			(5,000,000.0)	(5,000,000.0)	(5,000,000.0)
Goodwill, beginning balance			148,100,000	148,100,000	148,100,000
Additions			0	0
Impairments			0	0
Disposals			0	0
Decrease from reclassification to assets held for sale			0
Goodwill, ending balance	148,100,000	148,100,000	148,100,000	148,100,000	148,100,000
Accumulated impairment, ending balance	(5,000,000.0)	(5,000,000.0)	(5,000,000.0)	(5,000,000.0)	(5,000,000.0)
Goodwill gross, ending balance	153,100,000	153,100,000	153,100,000	153,100,000	153,100,000
Liquid Alpha
Goodwill [Roll Forward]
Goodwill gross, beginning balance			133,300,000	133,300,000	133,300,000
Accumulated impairment, beginning balance			(27,100,000)	(27,100,000)	(27,100,000)
Goodwill, beginning balance			106,200,000	106,200,000	106,200,000
Additions			0	0
Impairments			(16,400,000)	0
Disposals			0	0
Decrease from reclassification to assets held for sale			(76,100,000)
Goodwill, ending balance	13,700,000	106,200,000	13,700,000	106,200,000	106,200,000
Accumulated impairment, ending balance	(43,500,000)	(27,100,000)	(43,500,000)	(27,100,000)	(27,100,000)
Goodwill gross, ending balance	$ 57,200,000	$ 133,300,000	$ 57,200,000	$ 133,300,000	$ 133,300,000