Basis of Presentation and Significant Accounting Policies - Schedule of Changes in LFPB due to Adoption of ASU 2018-12 (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Whole Life
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions		$ (0.6)	$ 5.2	$ 2.8
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 at September 30, 2023	234.5	216.4	317.2	330.3
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]
Changes in cash flow assumptions		(16.7)	21.5	(4.8)
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 at September 30, 2023	111.1	97.1	112.1	114.2
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]
Changes in cash flow assumptions		5.0	11.0	(3.6)
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 at September 30, 2023	810.3	798.5	1,097.0	1,189.3
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]
Changes in cash flow assumptions		(0.2)	2.0	0.0
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 at September 30, 2023	56.2	49.6	73.0	69.3
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]
Changes in cash flow assumptions		8.9	13.8	(3.0)
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 at September 30, 2023	241.6	261.0	359.1	415.5
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	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 Future Policy Benefit [Roll Forward]
Changes in cash flow assumptions		0.0	0.0	0.0
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 at September 30, 2023	$ 100.6	$ 100.1	$ 124.7	136.5
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