Long-Duration Insurance Contracts - Amounts Recognized in the Statement of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	$ 239.0	$ 236.8
Interest expense	75.7	75.8
Whole Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	28.1	25.9
Interest expense	11.8	11.3
Term Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	45.2	42.4
Interest expense	4.8	4.4
Experience life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	32.1	33.5
Interest expense	43.7	44.2
Limited-pay whole life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	7.2	8.1
Interest expense	2.6	2.4
Supplemental health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	120.3	121.2
Interest expense	8.4	9.1
SPIA (life contingent)
Liability for Future Policy Benefit, by Product Segment [Line Items]
Gross premiums or assessments	6.1	5.7
Interest expense	$ 4.4	$ 4.4