Long-Duration Insurance Contracts - Balances of and Changes in the LFPB (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021	Dec. 31, 2020
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance	$ 1,761.8	$ 1,718.0	[1]
Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance		0.0
Whole Life
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	215.1	183.0		$ 176.5
Beginning balance at original discount rate	245.9	157.6		143.5
Beginning period (including reserves)				260.7
Beginning balance at original discount rate (including reserves)		239.3
Change in cash flow assumptions		0.0		5.2	$ 2.4
Actual variances from expected experience		3.8		7.2	8.8
Adjusted balance		249.7		251.7	154.7
Issuances	10.8	12.5		13.3
Interest accruals	7.2	6.7		6.2
Net premiums collected	(20.6)	(25.0)		(16.6)
Ending balance at original discount rate	247.1	245.9		157.6
Effect of changes in discount rate assumptions	(23.9)	(30.8)		25.4
Ending balance	223.2	215.1		183.0
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	493.6	500.7		507.1
Beginning balance (including reserves)		660.4
Beginning balance at original discount rate	592.1	581.9		386.3	362.5
Beginning balance at original discount rate (including reserves)		566.1
Changes in cash flow assumptions		(0.6)		5.2	2.8
Actual variances from expected experience		4.0		7.7	8.7
Adjusted balance		585.3		579.0	374.0
Issuances	10.7	12.4		13.3
Interest accruals	19.0	18.0		17.1
Benefit payments	(22.9)	(27.5)		(18.1)
Ending balance at original discount rate	592.1	581.9		386.3
Effect of changes in discount rate assumptions	(70.1)	(88.3)		114.4
Ending balance	522.0	493.6		500.7
Ending balance (including reserves)				660.4
Liability for Future Policy Benefit, before Reinsurance	298.8	278.4		317.7	330.6
Less: Reinsurance recoverable	(64.3)	(63.1)		(0.5)	(0.1)
Less: Change in discount rate assumptions					(0.2)
Net liability for future policy benefits, after reinsurance recoverable	234.5	215.3
Impact of flooring on net liability for future policy benefits	0.0	1.1
Net liability for future policy benefits at September 30, 2023	234.5	216.4		317.2	330.3
Whole Life | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					218.7
Whole Life | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					111.5
Whole Life | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.4
Whole Life | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Whole Life | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Term Life
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	234.7	253.1		244.1
Beginning balance at original discount rate	265.4	221.1		200.8
Beginning period (including reserves)				264.4
Beginning balance at original discount rate (including reserves)		235.4
Change in cash flow assumptions		(16.8)		18.7	(4.5)
Actual variances from expected experience		(2.7)		(4.2)	6.9
Adjusted balance		245.9		249.9	203.2
Issuances	25.2	28.0		29.8
Interest accruals	10.3	9.0		7.9
Net premiums collected	(24.8)	(21.5)		(19.8)
Ending balance at original discount rate	256.6	265.4		221.1
Effect of changes in discount rate assumptions	(16.6)	(30.7)		32.0
Ending balance	240.0	234.7		253.1
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	347.0	370.6		364.7
Beginning balance (including reserves)		411.5
Beginning balance at original discount rate	405.4	401.0		319.5	294.0
Beginning balance at original discount rate (including reserves)		360.0
Changes in cash flow assumptions		(16.7)		21.5	(4.8)
Actual variances from expected experience		1.3		(4.7)	7.2
Adjusted balance		385.6		376.8	296.4
Issuances	25.8	28.3		29.8
Interest accruals	15.2	14.4		12.0
Benefit payments	(21.2)	(18.5)		(18.7)
Ending balance at original discount rate	405.4	401.0		319.5
Effect of changes in discount rate assumptions	(35.3)	(54.0)		51.1
Ending balance	370.1	347.0		370.6
Ending balance (including reserves)				411.5
Liability for Future Policy Benefit, before Reinsurance	130.2	112.2		117.6	120.5
Less: Reinsurance recoverable	(19.1)	(15.3)		(5.5)	(5.4)
Less: Change in discount rate assumptions					(0.9)
Net liability for future policy benefits, after reinsurance recoverable	111.1	96.9
Impact of flooring on net liability for future policy benefits	0.0	0.2
Net liability for future policy benefits at September 30, 2023	111.1	97.1		112.1	114.2
Term Life | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					93.2
Term Life | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					27.3
Term Life | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Term Life | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Term Life | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Experience life
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	68.3	74.6		78.0
Beginning balance at original discount rate	65.5	55.8		55.2
Beginning period (including reserves)				74.6
Beginning balance at original discount rate (including reserves)		55.9
Change in cash flow assumptions		3.7		9.1	(3.3)
Actual variances from expected experience		0.7		3.0	6.3
Adjusted balance		69.9		68.0	58.2
Issuances	0.0	0.0		0.0
Interest accruals	3.7	3.3		3.2
Net premiums collected	(6.6)	(5.8)		(5.6)
Ending balance at original discount rate	67.0	65.5		55.8
Effect of changes in discount rate assumptions	4.7	2.8		18.8
Ending balance	71.7	68.3		74.6
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	867.5	1,172.7		1,269.3
Beginning balance (including reserves)		1,172.7
Beginning balance at original discount rate	797.5	805.2		802.5	813.5
Beginning balance at original discount rate (including reserves)		802.6
Changes in cash flow assumptions		5.0		11.0	(3.6)
Actual variances from expected experience		1.1		3.6	6.6
Adjusted balance		811.3		817.2	816.5
Issuances	0.0	0.0		0.0
Interest accruals	47.4	47.4		47.9
Benefit payments	(61.2)	(59.4)		(61.9)
Ending balance at original discount rate	797.5	805.2		802.5
Effect of changes in discount rate assumptions	85.5	62.3		370.2
Ending balance	883.0	867.5		1,172.7
Ending balance (including reserves)				1,172.7
Liability for Future Policy Benefit, before Reinsurance	811.3	799.3		1,098.1	1,191.3
Less: Reinsurance recoverable	(1.0)	(0.8)		(1.1)	(1.3)
Less: Change in discount rate assumptions					(0.7)
Net liability for future policy benefits, after reinsurance recoverable	810.3	798.5
Impact of flooring on net liability for future policy benefits	0.0	0.0
Net liability for future policy benefits at September 30, 2023	810.3	798.5		1,097.0	1,189.3
Experience life | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					758.3
Experience life | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					433.0
Experience life | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Experience life | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Experience life | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Limited-pay whole life
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	29.7	29.7		25.4
Beginning balance at original discount rate	32.4	27.2		22.0
Beginning period (including reserves)				29.7
Beginning balance at original discount rate (including reserves)		27.2
Change in cash flow assumptions		(0.2)		2.0	0.0
Actual variances from expected experience		1.0		1.6	1.0
Adjusted balance		33.2		30.8	23.0
Issuances	4.3	6.3		10.2
Interest accruals	1.2	1.1		0.8
Net premiums collected	(4.8)	(5.8)		(6.8)
Ending balance at original discount rate	33.9	32.4		27.2
Effect of changes in discount rate assumptions	(1.7)	(2.7)		2.5
Ending balance	32.2	29.7		29.7
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	79.4	102.9		95.0
Beginning balance (including reserves)		102.9
Beginning balance at original discount rate	105.6	98.6		86.6	73.4
Beginning balance at original discount rate (including reserves)		86.6
Changes in cash flow assumptions		(0.2)		2.0	0.0
Actual variances from expected experience		1.0		1.4	1.1
Adjusted balance		99.4		90.0	74.5
Issuances	4.3	6.4		10.2
Interest accruals	3.9	3.4		2.9
Benefit payments	(2.0)	(1.2)		(1.0)
Ending balance at original discount rate	105.6	98.6		86.6
Effect of changes in discount rate assumptions	(16.0)	(19.2)		16.3
Ending balance	89.6	79.4		102.9
Ending balance (including reserves)				102.9
Liability for Future Policy Benefit, before Reinsurance	57.4	49.6		73.2	69.5
Less: Reinsurance recoverable	(1.2)	0.0		(0.2)	(0.1)
Less: Change in discount rate assumptions					(0.1)
Net liability for future policy benefits, after reinsurance recoverable	56.2	49.6
Impact of flooring on net liability for future policy benefits	0.0	0.0
Net liability for future policy benefits at September 30, 2023	56.2	49.6		73.0	69.3
Limited-pay whole life | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					51.3
Limited-pay whole life | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					18.2
Limited-pay whole life | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Limited-pay whole life | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Limited-pay whole life | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Supplemental health
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	167.4	221.5		233.0
Beginning balance at original discount rate	205.1	217.5		218.2
Beginning period (including reserves)				226.7
Beginning balance at original discount rate (including reserves)		223.1
Change in cash flow assumptions		6.5		12.2	(1.8)
Actual variances from expected experience		(1.6)		(25.3)	6.3
Adjusted balance		210.0		210.0	222.7
Issuances	19.4	12.0		13.0
Interest accruals	6.0	5.9		5.9
Net premiums collected	(22.0)	(22.8)		(24.1)
Ending balance at original discount rate	213.4	205.1		217.5
Effect of changes in discount rate assumptions	(31.4)	(37.7)		4.0
Ending balance	182.0	167.4		221.5
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	431.7	580.6		626.9
Beginning balance (including reserves)		590.6
Beginning balance at original discount rate	517.9	537.1		572.6	589.1
Beginning balance at original discount rate (including reserves)		584.2
Changes in cash flow assumptions		8.9		13.8	(3.0)
Actual variances from expected experience		(2.4)		(30.0)	6.2
Adjusted balance		543.6		568.0	592.3
Issuances	19.4	12.0		13.0
Interest accruals	14.4	15.0		15.7
Benefit payments	(59.5)	(57.9)		(48.4)
Ending balance at original discount rate	517.9	537.1		572.6
Effect of changes in discount rate assumptions	(90.3)	(105.4)		8.0
Ending balance	427.6	431.7		580.6
Ending balance (including reserves)				590.6
Liability for Future Policy Benefit, before Reinsurance	245.6	264.4		359.1	415.5
Less: Reinsurance recoverable	(4.0)	(3.4)		0.0	0.0
Less: Change in discount rate assumptions					0.0
Net liability for future policy benefits, after reinsurance recoverable	241.6	261.0
Impact of flooring on net liability for future policy benefits	0.0	0.0
Net liability for future policy benefits at September 30, 2023	241.6	261.0		359.1	415.5
Supplemental health | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					392.5
Supplemental health | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					23.0
Supplemental health | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Supplemental health | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Supplemental health | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
Cancer
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at September 30, 2023	92.7	101.8		140.8	163.5
Accident
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at September 30, 2023	21.4	21.8		28.7	31.2
Disability
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at September 30, 2023	23.5	23.1		29.3	32.0
Other
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at September 30, 2023	104.0	114.3		160.3	188.8
SPIA (life contingent)
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	0.0	0.0		0.0
Beginning balance at original discount rate	0.0	0.0		0.0
Beginning period (including reserves)				0.0
Beginning balance at original discount rate (including reserves)		0.0
Change in cash flow assumptions		0.0		0.0	0.0
Actual variances from expected experience		0.0		0.0	0.0
Adjusted balance		0.0		0.0	0.0
Issuances	5.6	5.3		3.7
Interest accruals	0.0	0.0		0.0
Net premiums collected	(5.6)	(5.3)		(3.7)
Ending balance at original discount rate	0.0	0.0		0.0
Effect of changes in discount rate assumptions	0.0	0.0		0.0
Ending balance	0.0	0.0		0.0
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	103.3	124.7		136.5
Beginning balance (including reserves)		129.1
Beginning balance at original discount rate	111.4	113.4		111.6	115.9
Beginning balance at original discount rate (including reserves)		115.7
Changes in cash flow assumptions		0.0		0.0	0.0
Actual variances from expected experience		(0.8)		0.4	(0.4)
Adjusted balance		112.6		116.1	115.5
Issuances	6.3	5.3		3.7
Interest accruals	4.4	4.3		4.5
Benefit payments	(11.9)	(12.3)		(12.1)
Ending balance at original discount rate	111.4	113.4		111.6
Effect of changes in discount rate assumptions	(7.2)	(10.1)		13.1
Ending balance	104.2	103.3		124.7
Ending balance (including reserves)				129.1
Liability for Future Policy Benefit, before Reinsurance	104.2	103.3		124.7	136.5
Less: Reinsurance recoverable	(3.6)	(3.2)		0.0	0.0
Less: Change in discount rate assumptions					0.0
Net liability for future policy benefits, after reinsurance recoverable	100.6	100.1
Impact of flooring on net liability for future policy benefits	0.0	0.0
Net liability for future policy benefits at September 30, 2023	$ 100.6	$ 100.1		$ 124.7	136.5
SPIA (life contingent) | Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for Future Policy Benefit, before Reinsurance					115.9
SPIA (life contingent) | Change in discount rate assumptions
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					20.6
SPIA (life contingent) | Change in cash flow assumptions, effect of net premiums exceeding gross premiums
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
SPIA (life contingent) | Change in cash flow assumptions, effect of decrease of the DPL
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					0.0
SPIA (life contingent) | Adjustment for removal of related balances in AOCI
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions					$ 0.0

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements