GOVERNMENT AND GOVERNMENT-BACKED SECURITIES - Schedule of Purchases and Sales of Agency and Treasury Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Treasury
Purchases
Face	$ 25.0	$ 1,000.0	$ 4,825.0	$ 1,000.0
Purchase price	24.7	973.8	4,798.6	973.8
Sales
Face	3,000.0	0.0	3,000.0	0.0
Amortized cost	2,976.3	0.0	2,976.3	0.0
Sale price	2,957.5	0.0	2,957.5	0.0
Realized gain (loss)	(18.8)	0.0	(18.8)	0.0
Agency
Purchases
Face	0.0	0.0	1,287.0	2,162.4
Purchase price	0.0	0.0	1,255.9	2,154.4
Sales
Face	0.0	0.0	0.0	1,462.4
Amortized cost	0.0	0.0	0.0	1,442.8
Sale price	0.0	0.0	0.0	1,395.9
Realized gain (loss)	$ 0.0	$ 0.0	$ 0.0	$ (46.9)