Basis of Presentation and Significant Accounting Policies - Schedule of Changes in LFPB due to Adoption of ASU 2018-12 (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 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 benefits	$ 1,613.4		$ 1,718.0
Previously Reported
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits
Whole Life
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	262.7		278.4	$ 317.7	$ 330.6
Changes in cash flow assumptions		$ 0.0	0.0	5.2	2.8
Less: Reinsurance recoverable	(58.1)		(63.1)	(0.5)	(0.1)
Less: Change in discount rate assumptions					(0.2)
Net liability for future policy benefits at September 30, 2023	205.1		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 benefits					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
Term Life
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	112.4		112.2	117.6	120.5
Changes in cash flow assumptions		0.0	(5.6)	21.5	(4.8)
Less: Reinsurance recoverable	(16.9)		(15.3)	(5.5)	(5.4)
Less: Change in discount rate assumptions					(0.9)
Net liability for future policy benefits at September 30, 2023	95.6		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 benefits					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
Experience Life
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	743.2		799.3	1,098.1	1,191.3
Changes in cash flow assumptions		0.0	0.0	11.0	(3.6)
Less: Reinsurance recoverable	(0.6)		(0.8)	(1.1)	(1.3)
Less: Change in discount rate assumptions					(0.7)
Net liability for future policy benefits at September 30, 2023	742.6		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 benefits					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
Limited Pay Whole Life
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	48.2		49.6	73.2	69.5
Changes in cash flow assumptions		0.0	0.0	2.0	0.0
Less: Reinsurance recoverable	(0.1)		0.0	(0.2)	(0.1)
Less: Change in discount rate assumptions					(0.1)
Net liability for future policy benefits at September 30, 2023	48.1		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 benefits					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
Supplemental health
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	234.0		264.4	359.1	415.5
Changes in cash flow assumptions		0.0	0.0	13.8	(3.0)
Less: Reinsurance recoverable	(3.5)		(3.4)	0.0	0.0
Less: Change in discount rate assumptions					0.0
Net liability for future policy benefits at September 30, 2023	230.5		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 benefits					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
Cancer
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Net liability for future policy benefits at September 30, 2023	88.5		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	20.0		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	21.6		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	100.4		114.3	160.3	188.8
SPIA (life contingent)
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Liability for future policy benefits	98.2		103.3	124.7	136.5
Changes in cash flow assumptions		$ 0.0	0.0	0.0	0.0
Less: Reinsurance recoverable	(3.3)		(3.2)	0.0	0.0
Less: Change in discount rate assumptions					0.0
Net liability for future policy benefits at September 30, 2023	$ 94.9		$ 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 benefits					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