Borrowings - Partnership's Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Borrowing Outstanding	$ 2,296.7	$ 2,097.6
Carrying Value	2,271.7	2,071.6
CLO Term Loans
Debt Instrument [Line Items]
Borrowing Outstanding	421.7	222.6
Carrying Value	418.1	219.0
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Borrowing Outstanding	600.0	600.0
Carrying Value	$ 600.6	600.6
Interest rate	5.625%
5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Borrowing Outstanding	$ 350.0	350.0
Carrying Value	$ 346.3	346.1
Interest rate	5.65%
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Borrowing Outstanding	$ 425.0	425.0
Carrying Value	$ 422.0	421.6
Interest rate	3.50%
Subordinated Notes 4.625% Due 2061
Debt Instrument [Line Items]
Borrowing Outstanding	$ 500.0	500.0
Carrying Value	$ 484.7	$ 484.3
Interest rate	4.625%