INVESTMENTS - SUMMARY OF FIXED MATURITY SECURITIES AVAILABLE FOR SALE (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 23,384.2
Estimated fair value	$ 20,353.4	$ 24,805.4	$ 21,293.1
Percentage of total estimated fair value	100.00%
NAIC designation 1
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 14,205.4
Estimated fair value	$ 12,385.7
Percentage of total estimated fair value	60.90%
NAIC designation 2
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 8,407.1
Estimated fair value	$ 7,294.9
Percentage of total estimated fair value	35.80%
Total NAIC 1 and 2 (investment grade)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 22,612.5
Estimated fair value	$ 19,680.6
Percentage of total estimated fair value	96.70%
NAIC designation 3
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 606.9
Estimated fair value	$ 535.8
Percentage of total estimated fair value	2.60%
NAIC designation 4
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 145.5
Estimated fair value	$ 125.9
Percentage of total estimated fair value	0.60%
NAIC designation 5
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 18.3
Estimated fair value	$ 11.1
Percentage of total estimated fair value	0.10%
NAIC designation 6
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1.0
Estimated fair value	$ 0.0
Percentage of total estimated fair value	0.00%
Total NAIC 3,4,5 and 6 (below-investment grade)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 771.7
Estimated fair value	$ 672.8
Percentage of total estimated fair value	3.30%