SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ADOPTION (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale	$ 20,353.4	$ 24,805.4		$ 21,293.1
Mortgage loans	1,411.9	1,218.6		1,559.4
Investments held by variable interest entities	1,077.6	1,199.6		1,178.7
Income tax assets, net	1,165.5	118.3		437.5
Reinsurance receivables	4,241.7	4,354.3		4,781.7
Total assets	33,339.2	36,204.4		33,613.1
Retained earnings	1,459.0	1,127.2		517.9
Total shareholders' equity	$ 1,400.8	$ 5,259.7	$ 5,484.2	4,659.2	$ 4,677.0
ASU 2016-13 Credit Losses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale				(2.1)
Mortgage loans				(6.7)
Investments held by variable interest entities				(9.9)
Income tax assets, net				4.9
Reinsurance receivables				(4.0)
Total assets				(17.8)
Retained earnings				(17.8)
Total shareholders' equity				$ (17.8)
Amounts prior to effect of adoption of authoritative guidance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Fixed maturities, available for sale					21,295.2
Mortgage loans					1,566.1
Investments held by variable interest entities					1,188.6
Income tax assets, net					432.6
Reinsurance receivables					4,785.7
Total assets					33,630.9
Retained earnings					535.7
Total shareholders' equity					$ 4,677.0