INVESTMENTS - SCHEDULE OF INVESTMENTS CLASSIFIED BY CONTRACTUAL MATURITY DATE (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Amortized cost
Due in one year or less	$ 112.0
Due after one year through five years	1,913.7
Due after five years through ten years	2,098.9
Due after ten years	12,629.4
Subtotal	16,754.0
Structured securities	6,630.2
Total	23,384.2
Estimated fair value
Due in one year or less	110.8
Due after one year through five years	1,790.2
Due after five years through ten years	1,910.4
Due after ten years	10,523.2
Subtotal	14,334.6
Structured securities	6,018.8
Total	$ 20,353.4	$ 24,805.4	$ 21,293.1