Liability for Future Policyholder Benefits - Changes in Premiums, Discounts and Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefit, Weighted-Average Duration	14 years 7 months 6 days		14 years 10 months 24 days
Accounting Standards Update 2018-12
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefit, Expected Net Premium, before Reinsurance, after Discount Rate Change	$ 690.1	$ 722.4
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Cash Flow and Reinsurance	723.1	744.5
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Reinsurance, after Cash Flow Change			$ 700.0	$ 729.6
Present Value of Expected Future Policyholder benefits	3,488.2	3,699.5
Liability for Future Policy Benefit, Expected Future Policy Benefit, Original Discount Rate, before Cash Flow and Reinsurance	3,865.7	3,916.9
Liability for Future Policy Benefit, Expected Future Benefit, Original Discount Rate, before Reinsurance, after Cash Flow Change			3,841.4	3,906.5
Liability for Future Policy Benefit, after Reinsurance	2,798.1	2,977.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits	10,225.9	10,177.7
Term Life Insurance | Accounting Standards Update 2018-12
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefit, Expected Net Premium, before Reinsurance, after Discount Rate Change			675.4	688.6
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Cash Flow and Reinsurance			694.7	728.9
Liability for Future Policy Benefit, Expected Net Premium, Cumulative Increase (Decrease) from Cash Flow Change	0.0	0.0
Liability for Future Policy Benefit, Expected Net Premium, Cumulative Increase (Decrease) of Actual Variance from Expected Experience			5.3	0.7
Liability for Future Policy Benefit, Expected Net Premium, Issuance	38.2	31.8
Liability for Future Policy Benefit, Expected Net Premium, Interest Income	7.6	7.0
Liability for Future Policy Benefit, Expected Net Premium, Net Premium Collected	(22.7)	(23.9)
AOCI, Liability for Future Policy Benefit, Expected Net Premium, before Tax	(33.0)	(22.1)
Present Value of Expected Future Policyholder benefits			3,613.2	3,561.0
Liability for Future Policy Benefit, Expected Future Policy Benefit, Original Discount Rate, before Cash Flow and Reinsurance			3,835.9	3,906.2
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change	0.0	0.0
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) of Actual Variance from Expected Experience			$ 5.5	$ 0.3
Liability for Future Policy Benefit, Expected Future Policy Benefit, Issuance	38.2	31.8
Liability for Future Policy Benefit, Expected Future Policy Benefit, Interest Expense	42.6	42.4
Liability for Future Policy Benefit, Expected Future Policy Benefit, Benefit Payment	(56.5)	(63.8)
AOCI, Liability for Future Policy Benefit, Expected Future Policy Benefit, before Tax	(377.5)	(217.4)
Net Liability for Future Policyholder Benefits	2,798.1	2,977.1
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance	$ 0.0	$ 0.0