Long-Duration Insurance Contracts - Undiscounted and Discounted Expected Gross Premiums and Expected Future Benefits and Expenses (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	$ 476.1	$ 478.8
Expected future benefits and expenses, undiscounted	1,157.9	1,152.8
Expected future gross premiums, discounted	328.9	325.0
Expected future benefits and expenses, discounted	598.2	592.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	703.7	689.0
Expected future benefits and expenses, undiscounted	708.0	682.7
Expected future gross premiums, discounted	456.4	449.4
Expected future benefits and expenses, discounted	417.4	405.4
Experience Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	512.5	530.0
Expected future benefits and expenses, undiscounted	1,669.4	1,703.1
Expected future gross premiums, discounted	287.2	296.1
Expected future benefits and expenses, discounted	788.4	797.5
Limited Pay Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	67.4	64.7
Expected future benefits and expenses, undiscounted	253.7	244.9
Expected future gross premiums, discounted	50.8	49.1
Expected future benefits and expenses, discounted	108.8	105.6
Supplemental health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	1,624.2	1,624.1
Expected future benefits and expenses, undiscounted	710.5	719.4
Expected future gross premiums, discounted	1,188.1	1,192.5
Expected future benefits and expenses, discounted	505.7	517.9
Life Contingent SPIA Retirement Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	0.0	0.0
Expected future benefits and expenses, undiscounted	152.9	156.1
Expected future gross premiums, discounted	0.0	0.0
Expected future benefits and expenses, discounted	$ 108.8	$ 111.4