Borrowings - Partnership's Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Borrowing Outstanding	$ 2,306.7	$ 2,296.7
Carrying Value	2,281.0	2,271.7
CLO Term Loans
Debt Instrument [Line Items]
Borrowing Outstanding	431.7	421.7
Carrying Value	426.4	418.1
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Borrowing Outstanding	425.0	425.0
Carrying Value	$ 422.5	422.0
Interest rate	3.50%
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	$ 346.4	346.3
Interest rate	5.65%
4.625% Subordinated Notes Due 5/15/2061
Debt Instrument [Line Items]
Borrowing Outstanding	$ 500.0	500.0
Carrying Value	$ 485.1	$ 484.7
Interest rate	4.625%