Segment Information - Schedule of Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	$ 313.0	$ 298.3
Net investment income	110.7	115.9
Other segment income	7.8	5.5
Total segment revenues	431.5	419.7
Benefits and claims expenses (excluding catastrophe losses)	145.5	149.1
Catastrophe losses	11.3	16.4
Loss adjustment expenses	16.9	16.3
Interest credited	53.8	52.8
Operating & admin expenses	82.0	76.9
Commissions expense	39.2	37.9
Taxes, licenses and fees	7.8	7.4
Deferred policy acquisition costs	(31.5)	(31.4)
Deferred policy acquisition cost amortization	32.3	29.6
Interest expense	9.5	8.9
Total segment expenses	366.8	364.0
Pretax profit (loss)	64.7	55.7
Income tax expense	12.2	11.0
Segment profit (loss) (Core earnings)	52.5	44.7
Net investment losses (after-tax)	(1.7)	(2.6)
Non-core income adjustments (after-tax)	(9.6)	(3.9)
Net income	41.2	38.2
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	203.2	192.7
Net investment income	14.6	11.7
Other segment income	2.0	1.3
Total segment revenues	219.8	205.7
Benefits and claims expenses (excluding catastrophe losses)	85.9	85.6
Catastrophe losses	11.3	16.4
Loss adjustment expenses	16.9	16.3
Interest credited	0.0	0.0
Operating & admin expenses	33.4	33.4
Commissions expense	16.4	16.9
Taxes, licenses and fees	5.2	4.9
Deferred policy acquisition costs	(23.7)	(24.3)
Deferred policy acquisition cost amortization	25.8	23.1
Interest expense	0.0	0.0
Total segment expenses	171.2	172.3
Pretax profit (loss)	48.6	33.4
Income tax expense	9.6	6.6
Segment profit (loss) (Core earnings)	39.0	26.8
Net investment losses (after-tax)	0.0	0.0
Non-core income adjustments (after-tax)	0.0	0.0
Net income	39.0	26.8
Life & Retirement
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	39.4	38.5
Net investment income	87.0	89.1
Other segment income	5.5	4.8
Total segment revenues	131.9	132.4
Benefits and claims expenses (excluding catastrophe losses)	31.5	36.6
Catastrophe losses	0.0	0.0
Loss adjustment expenses	0.0	0.0
Interest credited	52.4	51.6
Operating & admin expenses	25.1	23.9
Commissions expense	11.0	9.7
Taxes, licenses and fees	1.0	0.9
Deferred policy acquisition costs	(6.4)	(6.1)
Deferred policy acquisition cost amortization	6.0	6.0
Interest expense	0.0	0.0
Total segment expenses	120.6	122.7
Pretax profit (loss)	11.3	9.7
Income tax expense	2.1	1.8
Segment profit (loss) (Core earnings)	9.2	7.9
Net investment losses (after-tax)	0.0	0.0
Non-core income adjustments (after-tax)	(2.1)	(1.1)
Net income	7.1	6.8
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	70.4	67.1
Net investment income	8.9	9.4
Other segment income	(0.6)	(1.1)
Total segment revenues	78.7	75.4
Benefits and claims expenses (excluding catastrophe losses)	28.1	26.9
Catastrophe losses	0.0	0.0
Loss adjustment expenses	0.0	0.0
Interest credited	1.4	1.2
Operating & admin expenses	20.6	17.3
Commissions expense	11.8	11.3
Taxes, licenses and fees	1.5	1.4
Deferred policy acquisition costs	(1.4)	(1.0)
Deferred policy acquisition cost amortization	0.5	0.5
Interest expense	0.0	0.0
Total segment expenses	62.5	57.6
Pretax profit (loss)	16.2	17.8
Income tax expense	3.6	3.8
Segment profit (loss) (Core earnings)	12.6	14.0
Net investment losses (after-tax)	0.0	0.0
Non-core income adjustments (after-tax)	(2.7)	(2.8)
Net income	9.9	11.2
Corporate & Other
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	0.0	0.0
Net investment income	0.2	5.7
Other segment income	0.9	0.5
Total segment revenues	1.1	6.2
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	2.9	2.3
Commissions expense	0.0	0.0
Taxes, licenses and fees	0.1	0.2
Deferred policy acquisition costs	0.0	0.0
Deferred policy acquisition cost amortization	0.0	0.0
Interest expense	9.5	8.9
Total segment expenses	12.5	11.4
Pretax profit (loss)	(11.4)	(5.2)
Income tax expense	(3.1)	(1.2)
Segment profit (loss) (Core earnings)	(8.3)	(4.0)
Net investment losses (after-tax)	(1.7)	(2.6)
Non-core income adjustments (after-tax)	(4.8)	0.0
Net income	$ (14.8)	$ (6.6)