Segment Information - Schedule of Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information
Net premiums and contract charges earned	$ 298.3	$ 275.2
Net investment income	115.9	105.4
Other segment income	5.5	3.2
Total segment revenues	419.7	383.8
Benefits and claims expenses (excluding catastrophe losses)	149.1	145.1
Catastrophe losses	16.4	16.2
Loss adjustment expenses	16.3	17.4
Interest credited	52.8	52.9
Operating & admin expenses	76.9	72.2
Commissions expense	37.9	32.8
Taxes, licenses and fees	7.4	7.4
Deferred policy acquisition costs	(31.4)	(27.9)
Deferred policy acquisition cost amortization	29.6	27.0
Interest expense	8.9	8.7
Total segment expenses	364.0	351.8
Pretax profit (loss)	55.7	32.0
Income tax expense	11.0	6.2
Segment profit (loss) (Core earnings)	44.7	25.8
Net investment losses (after-tax)	(2.6)	1.7
Change in MRB (after-tax)	(1.1)	1.8
Intangible asset amortization (after-tax)	(2.8)	(2.8)
Net income	38.2	26.5
Property & Casualty
Segment Reporting Information
Net premiums and contract charges earned	192.7	173.2
Net investment income	11.7	12.3
Other segment income	1.3	0.7
Total segment revenues	205.7	186.2
Benefits and claims expenses (excluding catastrophe losses)	85.6	91.1
Catastrophe losses	16.4	16.2
Loss adjustment expenses	16.3	17.4
Interest credited	0.0	0.0
Operating & admin expenses	33.4	30.3
Commissions expense	16.9	13.9
Taxes, licenses and fees	4.9	4.7
Deferred policy acquisition costs	(24.3)	(20.3)
Deferred policy acquisition cost amortization	23.1	19.8
Interest expense	0.0	0.0
Total segment expenses	172.3	173.1
Pretax profit (loss)	33.4	13.1
Income tax expense	6.6	2.5
Segment profit (loss) (Core earnings)	26.8	10.6
Net investment losses (after-tax)	0.0	0.0
Change in MRB (after-tax)	0.0	0.0
Intangible asset amortization (after-tax)	0.0	0.0
Net income	26.8	10.6
Life & Retirement
Segment Reporting Information
Net premiums and contract charges earned	38.5	37.8
Net investment income	89.1	85.8
Other segment income	4.8	4.8
Total segment revenues	132.4	128.4
Benefits and claims expenses (excluding catastrophe losses)	36.6	32.2
Catastrophe losses	0.0	0.0
Loss adjustment expenses	0.0	0.0
Interest credited	51.6	51.8
Operating & admin expenses	23.9	21.9
Commissions expense	9.7	9.5
Taxes, licenses and fees	0.9	1.0
Deferred policy acquisition costs	(6.1)	(6.7)
Deferred policy acquisition cost amortization	6.0	6.6
Interest expense	0.0	0.0
Total segment expenses	122.7	116.3
Pretax profit (loss)	9.7	12.1
Income tax expense	1.8	2.2
Segment profit (loss) (Core earnings)	7.9	9.9
Net investment losses (after-tax)	0.0	0.0
Change in MRB (after-tax)	(1.1)	1.8
Intangible asset amortization (after-tax)	0.0	0.0
Net income	6.8	11.7
Supplemental & Group Benefits
Segment Reporting Information
Net premiums and contract charges earned	67.1	64.2
Net investment income	9.4	7.8
Other segment income	(1.1)	(3.0)
Total segment revenues	75.4	69.0
Benefits and claims expenses (excluding catastrophe losses)	26.9	21.8
Catastrophe losses	0.0	0.0
Loss adjustment expenses	0.0	0.0
Interest credited	1.2	1.1
Operating & admin expenses	17.3	18.0
Commissions expense	11.3	9.4
Taxes, licenses and fees	1.4	1.5
Deferred policy acquisition costs	(1.0)	(0.9)
Deferred policy acquisition cost amortization	0.5	0.6
Interest expense	0.0	0.0
Total segment expenses	57.6	51.5
Pretax profit (loss)	17.8	17.5
Income tax expense	3.8	3.7
Segment profit (loss) (Core earnings)	14.0	13.8
Net investment losses (after-tax)	0.0	0.0
Change in MRB (after-tax)	0.0	0.0
Intangible asset amortization (after-tax)	(2.8)	(2.8)
Net income	11.2	11.0
Corporate & Other
Segment Reporting Information
Net premiums and contract charges earned	0.0	0.0
Net investment income	5.7	(0.5)
Other segment income	0.5	0.7
Total segment revenues	6.2	0.2
Benefits and claims expenses (excluding catastrophe losses)	0.0	0.0
Catastrophe losses	0.0	0.0
Loss adjustment expenses	0.0	0.0
Interest credited	0.0	0.0
Operating & admin expenses	2.3	2.0
Commissions expense	0.0	0.0
Taxes, licenses and fees	0.2	0.2
Deferred policy acquisition costs	0.0	0.0
Deferred policy acquisition cost amortization	0.0	0.0
Interest expense	8.9	8.7
Total segment expenses	11.4	10.9
Pretax profit (loss)	(5.2)	(10.7)
Income tax expense	(1.2)	(2.2)
Segment profit (loss) (Core earnings)	(4.0)	(8.5)
Net investment losses (after-tax)	(2.6)	1.7
Change in MRB (after-tax)	0.0	0.0
Intangible asset amortization (after-tax)	0.0	0.0
Net income	$ (6.6)	$ (6.8)