Borrowings - Debt Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Borrowing Outstanding	$ 1,997.3	$ 1,985.7
Carrying Value	1,987.1	1,976.3
CLO Borrowings
Debt Instrument [Line Items]
Borrowing Outstanding	350.0	324.9
Carrying Value	347.8	324.0
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Borrowing Outstanding	250.0	250.0
Carrying Value	249.4	249.3
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Borrowing Outstanding	600.0	600.0
Carrying Value	600.6	600.7
5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Borrowing Outstanding	350.0	350.0
Carrying Value	346.0	345.8
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Borrowing Outstanding	425.0	425.0
Carrying Value	421.0	420.7
Revolving Credit Facility | Global Credit Revolving Credit Facility
Debt Instrument [Line Items]
Borrowing Outstanding	22.3	35.8
Carrying Value	$ 22.3	$ 35.8