Goodwill and Intangible Assets - Changes in Goodwill (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill	$ 308.5	$ 308.5	$ 308.5	$ 308.5
Accumulated impairment	(50.3)	(33.9)	(33.9)	(33.9)
Beginning balance	274.6	274.6
Additions	0.0	0.0
Impairments	(16.4)	0.0
Disposals	0.0	0.0
Ending balance	258.2	274.6
Quant & Solutions
Goodwill [Line Items]
Goodwill	22.1	22.1	22.1	22.1
Accumulated impairment	(1.8)	(1.8)	(1.8)	(1.8)
Beginning balance	20.3	20.3
Additions	0.0	0.0
Impairments	0.0	0.0
Disposals	0.0	0.0
Ending balance	20.3	20.3
Alternatives
Goodwill [Line Items]
Goodwill	153.1	153.1	153.1	153.1
Accumulated impairment	(5.0)	(5.0)	(5.0)	(5.0)
Beginning balance	148.1	148.1
Additions	0.0	0.0
Impairments	0.0	0.0
Disposals	0.0	0.0
Ending balance	148.1	148.1
Liquid Alpha
Goodwill [Line Items]
Goodwill	133.3	133.3	133.3	133.3
Accumulated impairment	(43.5)	(27.1)	$ (27.1)	$ (27.1)
Beginning balance	106.2	106.2
Additions	0.0	0.0
Impairments	(16.4)	0.0
Disposals	0.0	0.0
Ending balance	$ 89.8	$ 106.2