Segments - Management Fee Revenue by Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Management fee revenue	$ 178.2	$ 196.4	$ 527.4	$ 609.8
Management fees
Segment Reporting Information [Line Items]
Management fee revenue	178.2	196.4	527.4	609.8
Management fees | U.S.
Segment Reporting Information [Line Items]
Management fee revenue	134.1	148.2	395.9	459.4
Management fees | Non-U.S.
Segment Reporting Information [Line Items]
Management fee revenue	$ 44.1	$ 48.2	$ 131.5	$ 150.4