Long-Duration Insurance Contracts - Undiscounted and Discounted Expected Gross Premiums and Expected Future Benefits and Expenses (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	$ 476.3	$ 478.8
Expected future benefits and expenses, undiscounted	1,160.3	1,152.8
Expected future gross premiums, discounted	328.6	325.0
Expected future benefits and expenses, discounted	599.9	592.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	708.5	689.0
Expected future benefits and expenses, undiscounted	717.4	682.7
Expected future gross premiums, discounted	458.9	449.4
Expected future benefits and expenses, discounted	422.2	405.4
Experience Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	504.0	530.0
Expected future benefits and expenses, undiscounted	1,654.6	1,703.1
Expected future gross premiums, discounted	282.9	296.1
Expected future benefits and expenses, discounted	785.0	797.5
Limited Pay Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	67.8	64.7
Expected future benefits and expenses, undiscounted	256.8	244.9
Expected future gross premiums, discounted	51.1	49.1
Expected future benefits and expenses, discounted	110.1	105.6
Supplemental health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	1,625.9	1,624.1
Expected future benefits and expenses, undiscounted	704.1	719.4
Expected future gross premiums, discounted	1,186.4	1,192.5
Expected future benefits and expenses, discounted	497.3	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	152.3	156.1
Expected future gross premiums, discounted	0.0	0.0
Expected future benefits and expenses, discounted	$ 108.4	$ 111.4