INVESTMENTS - SUMMARY OF INVESTMENTS BY CONTRACTUAL MATURITY (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Amortized cost
Due in one year or less	$ 123.4	$ 80.3
Due after one year through five years	1,935.7	1,147.4
Due after five years through ten years	2,125.3	1,458.4
Due after ten years	12,830.6	13,409.9
Subtotal	17,015.0	16,096.0
Structured securities	6,427.5	5,771.6
Amortized cost	23,442.5	21,867.6
Estimated fair value
Due in one year or less	122.6	80.5
Due after one year through five years	1,805.1	1,205.6
Due after five years through ten years	1,894.4	1,573.7
Due after ten years	10,583.8	15,913.3
Subtotal	14,405.9	18,773.1
Structured securities	5,895.2	6,032.3
Estimated fair value	$ 20,301.1	$ 24,805.4