Segments - Management Fee Revenue by Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Management fee revenue	$ 178.5	$ 207.5
Management fees
Segment Reporting Information [Line Items]
Management fee revenue	178.5	207.5
Management fees | U.S.
Segment Reporting Information [Line Items]
Management fee revenue	133.1	156.3
Management fees | Non-U.S.
Segment Reporting Information [Line Items]
Management fee revenue	$ 45.4	$ 51.2