Long-Duration Insurance Contracts - Balances of and Changes in the LFPB (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance	$ 1,702.1		$ 1,761.8
Whole Life Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	223.2
Beginning period (including reserves)				$ 215.1
Beginning balance at original discount rate	247.1
Beginning balance at original discount rate (including reserves)		$ 245.9
Change in cash flow assumptions			0.0	0.0
Actual variances from expected experience			0.3	3.8
Adjusted balance			247.4	249.7
Issuances	2.5	10.8
Interest accruals	1.9	7.2
Net premiums collected	(7.8)	(20.6)
Ending balance at original discount rate	244.0
Effect of changes in discount rate assumptions	(28.5)		(23.9)
Ending balance	215.5
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	522.0
Beginning balance (including reserves)		493.6
Beginning balance at original discount rate	592.0		592.1
Beginning balance at original discount rate (including reserves)		581.9
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	(0.6)
Actual variances from expected experience			0.5	4.0
Adjusted balance			592.6	585.3
Issuances	2.5	10.7
Interest accruals	4.8	19.0
Benefit payments	(7.9)	(22.9)
Ending balance at original discount rate	592.0
Effect of changes in discount rate assumptions	(88.2)		(70.1)
Ending balance	503.8
Liability for Future Policy Benefit, before Reinsurance	288.3		298.8
Less: Reinsurance recoverable	(62.5)		(64.3)
Net liability for future policy benefits, after reinsurance recoverable	225.8		234.5
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	225.8		234.5
Term Life Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	240.0
Beginning period (including reserves)				234.7
Beginning balance at original discount rate	256.6
Beginning balance at original discount rate (including reserves)		265.4
Change in cash flow assumptions			0.0	(16.8)
Actual variances from expected experience			0.3	(2.7)
Adjusted balance			256.9	245.9
Issuances	6.1	25.2
Interest accruals	2.6	10.3
Net premiums collected	(6.2)	(24.8)
Ending balance at original discount rate	259.4
Effect of changes in discount rate assumptions	(21.7)		(16.6)
Ending balance	237.7
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	370.1
Beginning balance (including reserves)		347.0
Beginning balance at original discount rate	411.1		405.4
Beginning balance at original discount rate (including reserves)		401.0
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	(16.7)
Actual variances from expected experience			1.0	1.3
Adjusted balance			406.4	385.6
Issuances	6.2	25.8
Interest accruals	3.9	15.2
Benefit payments	(5.4)	(21.2)
Ending balance at original discount rate	411.1
Effect of changes in discount rate assumptions	(45.0)		(35.3)
Ending balance	366.1
Liability for Future Policy Benefit, before Reinsurance	128.3		130.2
Less: Reinsurance recoverable	(19.0)		(19.1)
Net liability for future policy benefits, after reinsurance recoverable	109.3		111.1
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	109.3		111.1
Experience Life Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	71.7
Beginning period (including reserves)				68.3
Beginning balance at original discount rate	67.0
Beginning balance at original discount rate (including reserves)		65.5
Change in cash flow assumptions			0.0	3.7
Actual variances from expected experience			(0.1)	0.7
Adjusted balance			66.9	69.9
Issuances	0.0	0.0
Interest accruals	0.9	3.7
Net premiums collected	(1.7)	(6.6)
Ending balance at original discount rate	66.1
Effect of changes in discount rate assumptions	2.9		4.7
Ending balance	69.0
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	883.0
Beginning balance (including reserves)		867.5
Beginning balance at original discount rate	791.7		797.5
Beginning balance at original discount rate (including reserves)		805.2
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	5.0
Actual variances from expected experience			0.0	1.1
Adjusted balance			797.5	811.3
Issuances	0.0	0.0
Interest accruals	11.8	47.4
Benefit payments	(17.6)	(61.2)
Ending balance at original discount rate	791.7
Effect of changes in discount rate assumptions	57.9		85.5
Ending balance	849.6
Liability for Future Policy Benefit, before Reinsurance	780.6		811.3
Less: Reinsurance recoverable	(1.1)		(1.0)
Net liability for future policy benefits, after reinsurance recoverable	779.5		810.3
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	779.5		810.3
Limited Pay Whole Life Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	32.2
Beginning period (including reserves)				29.7
Beginning balance at original discount rate	33.9
Beginning balance at original discount rate (including reserves)		32.4
Change in cash flow assumptions			0.0	(0.2)
Actual variances from expected experience			0.2	1.0
Adjusted balance			34.1	33.2
Issuances	1.0	4.3
Interest accruals	0.3	1.2
Net premiums collected	(1.2)	(4.8)
Ending balance at original discount rate	34.2
Effect of changes in discount rate assumptions	(2.3)		(1.7)
Ending balance	31.9
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	89.6
Beginning balance (including reserves)		79.4
Beginning balance at original discount rate	107.3		105.6
Beginning balance at original discount rate (including reserves)		98.6
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	(0.2)
Actual variances from expected experience			0.1	1.0
Adjusted balance			105.7	99.4
Issuances	1.0	4.3
Interest accruals	1.0	3.9
Benefit payments	(0.4)	(2.0)
Ending balance at original discount rate	107.3
Effect of changes in discount rate assumptions	(20.1)		(16.0)
Ending balance	87.2
Liability for Future Policy Benefit, before Reinsurance	55.3		57.4
Less: Reinsurance recoverable	(1.1)		(1.2)
Net liability for future policy benefits, after reinsurance recoverable	54.2		56.2
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	54.2		56.2
Supplemental health
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	182.0
Beginning period (including reserves)				167.4
Beginning balance at original discount rate	213.4
Beginning balance at original discount rate (including reserves)		205.1
Change in cash flow assumptions			0.0	6.5
Actual variances from expected experience			0.2	(1.6)
Adjusted balance			213.6	210.0
Issuances	5.6	19.4
Interest accruals	1.6	6.0
Net premiums collected	(6.2)	(22.0)
Ending balance at original discount rate	214.6
Effect of changes in discount rate assumptions	(35.1)		(31.4)
Ending balance	179.5
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	427.6
Beginning balance (including reserves)		431.7
Beginning balance at original discount rate	510.5		517.9
Beginning balance at original discount rate (including reserves)		537.1
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	8.9
Actual variances from expected experience			0.1	(2.4)
Adjusted balance			518.0	543.6
Issuances	5.6	19.4
Interest accruals	3.6	14.4
Benefit payments	(16.7)	(59.5)
Ending balance at original discount rate	510.5
Effect of changes in discount rate assumptions	(98.9)		(90.3)
Ending balance	411.6
Liability for Future Policy Benefit, before Reinsurance	231.8		245.6
Less: Reinsurance recoverable	(3.8)		(4.0)
Net liability for future policy benefits, after reinsurance recoverable	228.0		241.6
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	228.0		241.6
Cancer
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at March 31, 2024	113.1		92.7
Accident
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at March 31, 2024	20.7		21.4
Disability
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at March 31, 2024	26.5		23.5
Other
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at March 31, 2024	122.6		104.0
Life Contingent SPIA Retirement Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	0.0
Beginning period (including reserves)				0.0
Beginning balance at original discount rate	0.0
Beginning balance at original discount rate (including reserves)		0.0
Change in cash flow assumptions			0.0	0.0
Actual variances from expected experience			0.0	0.0
Adjusted balance			0.0	0.0
Issuances	0.6	5.6
Interest accruals	0.0	0.0
Net premiums collected	(0.6)	(5.6)
Ending balance at original discount rate	0.0
Effect of changes in discount rate assumptions	0.0		0.0
Ending balance	0.0
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	104.2
Beginning balance (including reserves)		103.3
Beginning balance at original discount rate	110.3		111.4
Beginning balance at original discount rate (including reserves)		113.4
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change			0.0	0.0
Actual variances from expected experience			0.3	(0.8)
Adjusted balance			111.7	$ 112.6
Issuances	0.6	6.3
Interest accruals	1.1	4.4
Benefit payments	(3.1)	$ (11.9)
Ending balance at original discount rate	110.3
Effect of changes in discount rate assumptions	(9.0)		(7.2)
Ending balance	101.3
Liability for Future Policy Benefit, before Reinsurance	101.3		104.2
Less: Reinsurance recoverable	(3.5)		(3.6)
Net liability for future policy benefits, after reinsurance recoverable	97.8		100.6
Impact of flooring on net liability for future policy benefits	0.0		0.0
Net liability for future policy benefits at March 31, 2024	$ 97.8		$ 100.6