Liability for Future Policyholder Benefits - Changes in Premiums, Discounts and Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Mar. 31, 2025	Mar. 31, 2024
Liability for Future Policy Benefit, by Product Segment [Line Items]
Weighted-Average Liability Duration of the Liability for Future Policyholder Benefits (Years)	13 years 8 months 12 days	14 years 1 month 6 days	13 years 8 months 12 days	14 years 1 month 6 days
Effect of Retrospective Application of Accounting Standards Update 2018-12
Liability for Future Policy Benefit, by Product Segment [Line Items]
Adjusted Beginning of Period Balance	$ 697.7	$ 700.4	$ 697.7	$ 700.4	$ 701.9	$ 723.5
Ending Balance at Original Discount Rate	705.6	733.2	705.6	733.2
Balance, End of Period	683.1	692.6	683.1	692.6
Adjusted Beginning of Period Balance	3,828.8	3,840.1	3,828.8	3,840.1	3,830.2	3,864.4
Ending Balance at Original Discount Rate	3,834.3	3,874.5	3,834.3	3,874.5
Balance, End of Period	3,343.4	3,380.1	3,343.4	3,380.1
Net Liability for Future Policyholder Benefits, After Reinsurance Recoverable	2,660.3	2,688.1	2,660.3	2,688.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits, post-flooring	10,162.1	10,260.5	10,162.1	10,260.5
Term Life Insurance | Effect of Retrospective Application of Accounting Standards Update 2018-12
Liability for Future Policy Benefit, by Product Segment [Line Items]
Balance, Beginning of Period	668.2	690.1	646.1	675.4
Beginning Balance at Original Discount Rate	695.0	723.1	681.0	694.7
Effect of Changes in Cash Flow Assumptions	0.0	0.0	0.0	0.0
Effect of Actual Variances from Expected Experience	16.7	5.7	16.7	5.7	6.9	0.4
Issuances	20.3	25.3	40.7	63.5
Interest Accrual	7.5	7.9	14.8	15.5
Net Premiums Collected	(24.1)	(23.5)	(47.6)	(46.2)
Effect of Changes in Discount Rate Assumptions	(22.5)	(40.6)	(22.5)	(40.6)
Balance, Beginning of Period	3,333.5	3,488.2	3,295.9	3,613.2
Beginning Balance at Original Discount Rate	3,823.5	3,865.7	3,812.1	3,835.9
Effect of Changes in Cash Flow Assumptions	0.0	0.0	0.0	0.0
Effect of Actual Variances From Expected Experience	16.7	4.2	16.7	4.2	$ 6.7	$ (1.3)
Issuances	20.7	25.2	41.1	63.4
Interest Accrual	42.0	42.6	84.0	85.2
Benefit Payments	(58.6)	(57.7)	(119.6)	(114.2)
Effect of Changes in Discount Rate Assumptions	(490.9)	(494.4)	(490.9)	(494.4)
Net Liability for Future Policyholder Benefits, post-flooring	2,660.3	2,687.5	2,660.3	2,687.5
Cumulative impact of flooring the future Policyholder Benefits Reserve	0.0	0.6	0.0	0.6
Net Liability for Future Policyholder Benefits, post-flooring	2,660.3	2,688.1	2,660.3	2,688.1
Less: Reinsurance Recoverable	$ 0.0	$ 0.0	$ 0.0	$ 0.0