Liability for Future Policyholder Benefits - Reconciliation of the Net Liability for Future Benefits to Life and Health Insurance Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Insurance [Abstract]
Present Value of Expected Future Policyholder benefits	$ 3,613.2	$ 3,561.0	$ 4,933.1
Net Liability for Future Policyholder Benefits	2,937.8	2,872.4
Deferred Profit Liability	337.8	253.6	193.4	$ 112.7
A&H and Other Reserves	146.8	150.2
Total Life and Health Insurance Reserves	3,422.4	3,276.2
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits	2,937.8	2,872.4
Deferred Profit Liability	337.8	253.6	$ 193.4	112.7
A&H and Other Reserves	146.8	150.2
Total Life and Health Insurance Reserves	$ 3,422.4	$ 3,276.2
Effect of Retrospective Application of Accounting Standards Update 2018-12
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefit, Weighted-Average Duration	15 years 3 months 18 days	14 years 7 months 6 days	18 years 2 months 12 days
Term Life Insurance
Insurance [Abstract]
Gross Premiums or Assessments	$ 399.0	$ 392.1	$ 380.3
Interest Expense	141.3	142.1	144.9
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits	10,185.2	10,137.2
Liability for Future Policy Benefit, Expected Future Gross Premium, Undiscounted, before Reinsurance	4,107.9	4,436.8
Liability for Future Policy Benefit, Expected Future Gross Premium, Discounted, before Reinsurance	2,800.6	2,844.4
Gross Premiums or Assessments	399.0	392.1	380.3
Interest Expense	$ 141.3	$ 142.1	144.9
Liability for Future Policy Benefit, Weighted-Average Interest Accretion Rate	4.57%	4.60%
Liability for Future Policy Benefit, Current Weighted-Average Discount Rate	5.08%	5.30%
Term Life Insurance | Effect of Retrospective Application of Accounting Standards Update 2018-12
Insurance [Abstract]
Present Value of Expected Future Policyholder benefits		$ 3,561.0	4,933.1	$ 4,924.9
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits	$ 2,937.8	$ 2,872.4	$ 4,264.1