Borrowings - Debt Obligations (Details) - USD ($)	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2021	May 31, 2021
Debt Instrument [Line Items]
Borrowing Outstanding	$ 2,259,300,000	$ 2,306,700,000
Carrying Value	2,235,200,000	2,281,000,000
CLO Borrowings
Debt Instrument [Line Items]
Borrowing Outstanding	384,300,000	431,700,000
Carrying Value	$ 380,200,000	426,400,000
3.500% Senior Notes Due 9/19/2029
Debt Instrument [Line Items]
Interest rate (percent)	3.50%
Borrowing Outstanding	$ 425,000,000.0	425,000,000.0
Carrying Value	$ 422,700,000	422,500,000
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Interest rate (percent)	5.625%
Borrowing Outstanding	$ 600,000,000.0	600,000,000.0
Carrying Value	$ 600,500,000	600,600,000
5.650% Senior Notes Due 9/15/2048
Debt Instrument [Line Items]
Interest rate (percent)	5.65%
Borrowing Outstanding	$ 350,000,000.0	350,000,000.0
Carrying Value	$ 346,500,000	346,400,000
4.625% Subordinated Notes Due 5/15/2061
Debt Instrument [Line Items]
Interest rate (percent)	4.625%			4.625%
Borrowing Outstanding	$ 500,000,000.0	500,000,000.0	$ 65,000,000	$ 435,000,000
Carrying Value	$ 485,300,000	$ 485,100,000