RECENTLY ADOPTED ACCOUNTING STANDARDS - Transition Adjustments (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021	Jan. 01, 2021	Dec. 31, 2020
Present value of future profits
Balance	$ 197.6	$ 203.7	$ 223.2	$ 230.1	$ 259.6	$ 249.4
Deferred acquisition costs
Balance	1,811.3	1,770.9			1,485.8	1,027.8
Reinsurance receivables
Balance					4,728.4
Market risk benefit asset
Market risk benefit asset	57.8	65.3	16.9		2.5	0.0
Income tax assets, net
Balance	988.1	1,063.4			806.4	199.4
Policyholder Account Balance [Abstract]
Balance	15,302.9	15,234.2	14,725.6		12,367.7	12,540.6
Future policy benefits
Future policy benefits	11,623.3	11,240.2			15,704.9	11,744.2
Market risk benefit liability
Market risk benefit liability	17.6	11.3	63.8		114.8	0.0
Liability for policy and contract claims
Balance					91.7
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity	2,031.8	1,768.8	2,818.4	3,684.7		$ 5,484.2
Accumulated other comprehensive income (loss)
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity	(1,664.4)	(1,957.3)	(561.5)	373.7	106.3
Retained earnings
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity	$ 1,674.0	1,691.2	1,293.0	$ 1,125.6	621.4
Amount prior to adoption of ASU 2018-12
Present value of future profits
Balance		212.2			249.4
Deferred acquisition costs
Balance		1,913.4			1,027.8
Reinsurance receivables
Balance					4,584.3
Market risk benefit asset
Market risk benefit asset		0.0			0.0
Income tax assets, net
Balance		1,165.5			199.4
Policyholder Account Balance [Abstract]
Balance		14,858.3			12,540.6
Future policy benefits
Future policy benefits		11,809.1			11,744.2
Market risk benefit liability
Market risk benefit liability		0.0			0.0
Liability for policy and contract claims
Balance					561.8
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity		1,400.8	3,690.9
Amount prior to adoption of ASU 2018-12 | Accumulated other comprehensive income (loss)
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity					2,186.1
Amount prior to adoption of ASU 2018-12 | Retained earnings
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity					752.3
Effect of adoption
Present value of future profits
Balance		(8.5)
Unwinding amounts related to unrealized gains (losses)					10.2
Deferred acquisition costs
Balance		(142.5)
Unwinding amounts related to unrealized gains (losses)					458.0
Reinsurance receivables
Changes in measurement of assets					(81.6)
Change in discount rates					225.7
Market risk benefit asset
Market risk benefit asset		65.3
Changes in market benefit reserve basis					2.5
Income tax assets, net
Balance		(102.1)
Tax impacts of changes recognized in accumulated other comprehensive income					570.3
Tax impacts of changes recognized in retained earnings					36.7
Policyholder Account Balance [Abstract]
Balance		375.9
Changes in classification of liabilities					(172.9)
Future policy benefits
Future policy benefits		(568.9)
Unwinding amounts related to unrealized gains (losses)					(197.5)
Changes in remeasurement of future policy benefits					31.1
Changes in classification of liabilities					643.0
Reclass to market risk benefit liability					(66.6)
Change in discount rates					3,550.7
Market risk benefit liability
Market risk benefit liability		11.3
Reclass to market risk benefit liability					66.6
Changes in market risk benefit reserve basis					48.2
Liability for policy and contract claims
Changes in classification of liabilities					(470.1)
Statement of Stockholders' Equity [Abstract]
Total shareholders' equity		$ 368.0	$ (872.5)
Effect of adoption | Accumulated other comprehensive income (loss)
Statement of Stockholders' Equity [Abstract]
Unwinding amounts related to unrealized gains (losses)					665.7
Changes in measurement of assets and liabilities					0.0
Change in market risk benefit reserve basis					9.2
Tax impacts of changes recognized in retained earnings					0.0
Change in discount rates					(3,325.0)
Tax impacts of changes recognized in accumulated other comprehensive income					570.3
Effect of adoption | Retained earnings
Statement of Stockholders' Equity [Abstract]
Unwinding amounts related to unrealized gains (losses)					0.0
Changes in measurement of assets and liabilities					(112.7)
Change in market risk benefit reserve basis					(54.9)
Tax impacts of changes recognized in retained earnings					36.7
Change in discount rates					0.0
Tax impacts of changes recognized in accumulated other comprehensive income					$ 0.0