Goodwill and Intangible Assets - Changes in Goodwill (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 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
Disposals				0	0
Goodwill, ending balance	258,200,000		274,600,000	258,200,000	274,600,000
Accumulated impairment, ending balance	(50,300,000)		(33,900,000)	(50,300,000)	(33,900,000)
Goodwill gross, ending balance	308,500,000		308,500,000	308,500,000	308,500,000
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
Goodwill, ending balance	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)
Goodwill gross, ending balance	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
Goodwill, ending balance	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)
Goodwill gross, ending balance	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	0	$ (16,400,000)		(16,400,000)	0
Disposals				0	0
Goodwill, ending balance	89,800,000		106,200,000	89,800,000	106,200,000
Accumulated impairment, ending balance	(43,500,000)		(27,100,000)	(43,500,000)	(27,100,000)
Goodwill gross, ending balance	$ 133,300,000		$ 133,300,000	$ 133,300,000	$ 133,300,000