Liability for Future Policyholder Benefits - Changes in Premiums, Discounts and Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for Future Policy Benefit, Weighted-Average Duration	14 years 9 months 18 days	13 years 9 months 18 days	14 years 9 months 18 days	13 years 9 months 18 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	$ 733.2	$ 686.4	$ 733.2	$ 686.4
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Cash Flow and Reinsurance	745.2	742.5	745.2	742.5
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Reinsurance, after Cash Flow Change	699.0	710.3	699.0	710.3	$ 731.8		$ 738.8
Present Value of Expected Future Policyholder benefits	3,630.7	3,330.5	3,630.7	3,330.5
Liability for Future Policy Benefit, Expected Future Policy Benefit, Original Discount Rate, before Cash Flow and Reinsurance	3,886.8	3,909.1	3,886.8	3,909.1
Liability for Future Policy Benefit, Expected Future Benefit, Original Discount Rate, before Reinsurance, after Cash Flow Change	3,837.5	3,884.5	3,837.5	3,884.5	3,871.9		3,906.6
Liability for Future Policy Benefit, after Reinsurance	2,897.5	2,644.1	2,897.5	2,644.1
Term Life Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net Liability for Future Policyholder Benefits, post-flooring	10,274.0	10,181.5	10,274.0	10,181.5
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					692.6	$ 675.4	711.7	$ 688.6
Liability for Future Policy Benefit, Expected Net Premium, Original Discount Rate, before Cash Flow and Reinsurance					733.2	694.7	745.1	728.9
Liability for Future Policy Benefit, Expected Net Premium, Cumulative Increase (Decrease) from Cash Flow Change	0.0	0.0	0.0	0.0
Liability for Future Policy Benefit, Expected Net Premium, Cumulative Increase (Decrease) of Actual Variance from Expected Experience	4.3	(18.6)	4.3	(18.6)	(1.4)		(6.3)
Liability for Future Policy Benefit, Expected Net Premium, Issuance	29.4	19.7	92.9	81.6
Liability for Future Policy Benefit, Expected Net Premium, Interest Income	8.0	7.7	23.5	22.2
Liability for Future Policy Benefit, Expected Net Premium, Net Premium Collected	(24.0)	(23.7)	(70.2)	(71.6)
AOCI, Liability for Future Policy Benefit, Expected Net Premium, before Tax	(12.0)	(56.1)	(12.0)	(56.1)
Present Value of Expected Future Policyholder benefits					3,380.1	3,613.2	3,635.9	3,561.0
Liability for Future Policy Benefit, Expected Future Policy Benefit, Original Discount Rate, before Cash Flow and Reinsurance					3,874.5	$ 3,835.9	3,915.1	$ 3,906.2
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) from Cash Flow Change	0.0	0.0	0.0	0.0
Liability for Future Policy Benefit, Expected Future Policy Benefit, Cumulative Increase (Decrease) of Actual Variance from Expected Experience	1.6	(21.7)	1.6	(21.7)	$ (2.6)		$ (8.5)
Liability for Future Policy Benefit, Expected Future Policy Benefit, Issuance	29.5	18.9	92.9	80.8
Liability for Future Policy Benefit, Expected Future Policy Benefit, Interest Expense	43.0	43.0	128.2	128.2
Liability for Future Policy Benefit, Expected Future Policy Benefit, Benefit Payment	(57.6)	(59.4)	(171.8)	(184.4)
AOCI, Liability for Future Policy Benefit, Expected Future Policy Benefit, before Tax	(256.1)	(578.6)	(256.1)	(578.6)
Net Liability for Future Policyholder Benefits, post-flooring	2,897.5	2,644.1	2,897.5	2,644.1
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense	0.0	0.0	0.0	0.0
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustments	2,897.5	2,644.1	2,897.5	2,644.1
Liability for Future Policy Benefit, Reinsurance Recoverable, after Allowance	$ 0.0	$ 0.0	$ 0.0	$ 0.0