Long-Duration Insurance Contracts - Undiscounted and Discounted Expected Gross Premiums and Expected Future Benefits and Expenses (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	$ 473.5	$ 478.8
Expected future benefits and expenses, undiscounted	1,151.5	1,152.8
Expected future gross premiums, discounted	323.0	325.0
Expected future benefits and expenses, discounted	592.0	592.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	695.7	689.0
Expected future benefits and expenses, undiscounted	694.7	682.7
Expected future gross premiums, discounted	452.3	449.4
Expected future benefits and expenses, discounted	411.1	405.4
Experience Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	519.4	530.0
Expected future benefits and expenses, undiscounted	1,684.3	1,703.1
Expected future gross premiums, discounted	290.6	296.1
Expected future benefits and expenses, discounted	791.7	797.5
Limited Pay Whole Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	65.6	64.7
Expected future benefits and expenses, undiscounted	249.2	244.9
Expected future gross premiums, discounted	49.6	49.1
Expected future benefits and expenses, discounted	107.3	105.6
Supplemental health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Expected future gross premiums, undiscounted	1,619.2	1,624.1
Expected future benefits and expenses, undiscounted	712.7	719.4
Expected future gross premiums, discounted	1,187.4	1,192.5
Expected future benefits and expenses, discounted	510.5	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	155.0	156.1
Expected future gross premiums, discounted	0.0	0.0
Expected future benefits and expenses, discounted	$ 110.3	$ 111.4