Segments - Management Fee Revenue by Geographical Area (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Management fee revenue	$ 170.7	$ 205.9	$ 349.2	$ 413.4
Management fees
Segment Reporting Information [Line Items]
Management fee revenue	170.7	205.9	349.2	413.4
Management fees | U.S.
Segment Reporting Information [Line Items]
Management fee revenue	128.7	154.9	261.8	311.2
Management fees | Non-U.S.
Segment Reporting Information [Line Items]
Management fee revenue	$ 42.0	$ 51.0	$ 87.4	$ 102.2