Long-Duration Insurance Contracts - Reconciliation of LFPB to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	$ 1,613.4	$ 1,718.0
Whole life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	262.7	279.5
Term life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	112.4	112.4
Experience Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	743.2	799.3	$ 1,098.1	$ 1,191.3
Limited Pay Whole Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	48.2	49.6	73.2	69.5
Supplemental health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	234.0	264.4	359.1	415.5
SPIA (life contingent)
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	98.2	103.3	$ 124.7	$ 136.5
Limited-pay whole life DPL
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	3.9	3.2
SPIA (life contingent) DPL
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	1.2	0.8
Reconciling items
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits	$ 109.6	$ 105.5