Segment Information - Schedule of Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	$ 310.3	$ 289.1	$ 911.2	$ 845.2
Net investment income	119.6	113.0	356.5	326.8
Other segment income	5.3	6.2	15.0	14.0
Total segment revenues	435.2	408.3	1,282.7	1,186.0
Benefits and claims expenses (excluding catastrophe losses)	145.4	139.2	432.3	432.6
Catastrophe losses	9.9	34.0	56.0	91.1
Loss adjustment expenses	18.1	17.7	51.6	53.2
Interest credited	55.7	54.8	161.2	161.5
Operating & admin expenses	84.0	70.2	243.1	213.4
Commissions expense	42.8	36.8	118.3	105.1
Taxes, licenses and fees	8.0	8.0	24.7	22.9
Deferred policy acquisition costs	(38.6)	(32.0)	(102.1)	(91.1)
Deferred policy acquisition cost amortization	31.5	28.1	91.0	82.1
Interest expense	8.9	8.7	26.4	26.1
Total segment expenses	365.8	365.5	1,102.5	1,096.9
Pretax profit (loss)	69.4	42.8	180.2	89.1
Income tax expense	12.7	8.5	34.6	17.5
Segment profit (loss) (Core earnings)	56.7	34.3	145.6	71.6
Net investment losses (after-tax)	(2.7)	3.0	4.6	0.1
Non-core income adjustments (after-tax)	1.0		15.0
Change in MRB (after-tax)		(0.3)		1.6
Intangible asset amortization (after-tax)		(2.8)		(8.7)
Net income	58.3	34.3	125.9	64.6
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	204.7	187.3	594.7	539.7
Net investment income	14.7	8.8	41.4	30.6
Other segment income	0.6	0.6	2.6	1.9
Total segment revenues	220.0	196.7	638.7	572.2
Benefits and claims expenses (excluding catastrophe losses)	95.2	83.0	270.5	267.9
Catastrophe losses	9.9	34.0	56.0	91.1
Loss adjustment expenses	18.1	17.7	51.6	53.2
Interest credited	0.0	0.0	0.0	0.0
Operating & admin expenses	34.5	28.9	101.3	87.8
Commissions expense	21.2	16.8	54.6	46.4
Taxes, licenses and fees	5.8	5.6	16.8	15.2
Deferred policy acquisition costs	(30.2)	(24.3)	(79.2)	(67.3)
Deferred policy acquisition cost amortization	25.1	21.7	72.0	62.0
Interest expense	0.0	0.0	0.0	0.0
Total segment expenses	179.7	183.4	543.6	556.3
Pretax profit (loss)	40.4	13.3	95.0	15.9
Income tax expense	8.5	2.6	19.9	3.2
Segment profit (loss) (Core earnings)	31.8	10.7	75.1	12.7
Net investment losses (after-tax)	0.0	0.0	0.0	0.0
Non-core income adjustments (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	31.8	10.7	75.1	12.7
Life & Retirement
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	39.2	39.1	117.3	114.9
Net investment income	94.0	94.6	280.2	269.3
Other segment income	6.1	5.3	15.8	14.7
Total segment revenues	139.3	139.0	413.3	398.9
Benefits and claims expenses (excluding catastrophe losses)	31.2	34.9	93.0	98.1
Catastrophe losses	0.0	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0	0.0
Interest credited	54.2	53.6	157.3	158.0
Operating & admin expenses	25.0	21.5	72.9	66.4
Commissions expense	10.9	10.5	30.5	30.3
Taxes, licenses and fees	0.9	1.0	3.2	2.8
Deferred policy acquisition costs	(7.1)	(6.8)	(19.2)	(20.9)
Deferred policy acquisition cost amortization	5.9	5.9	17.6	18.6
Interest expense	0.0	0.0	0.0	0.0
Total segment expenses	121.1	120.6	355.3	353.3
Pretax profit (loss)	18.2	18.5	58.0	45.6
Income tax expense	3.1	3.4	10.4	8.2
Segment profit (loss) (Core earnings)	15.1	15.1	47.6	37.4
Net investment losses (after-tax)	0.0	0.0	0.0	0.0
Non-core income adjustments (after-tax)	(1.8)		3.9
Change in MRB (after-tax)		(0.3)		1.6
Intangible asset amortization (after-tax)		0.0		(0.2)
Net income	16.9	14.8	43.7	38.8
Supplemental & Group Benefits
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	66.4	62.7	199.2	190.6
Net investment income	11.1	10.1	31.1	28.4
Other segment income	(1.9)	(0.4)	(4.9)	(4.6)
Total segment revenues	75.6	72.4	225.4	214.4
Benefits and claims expenses (excluding catastrophe losses)	19.0	21.3	68.8	66.6
Catastrophe losses	0.0	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0	0.0
Interest credited	1.5	1.2	3.9	3.5
Operating & admin expenses	21.6	17.1	60.1	51.5
Commissions expense	10.7	9.5	33.2	28.4
Taxes, licenses and fees	1.1	1.3	4.2	4.3
Deferred policy acquisition costs	(1.3)	(0.9)	(3.7)	(2.9)
Deferred policy acquisition cost amortization	0.5	0.5	1.4	1.5
Interest expense	0.0	0.0	0.0	0.0
Total segment expenses	53.1	50.0	167.9	152.9
Pretax profit (loss)	22.5	22.4	57.5	61.5
Income tax expense	4.7	4.8	12.3	13.1
Segment profit (loss) (Core earnings)	17.8	17.6	45.2	48.4
Net investment losses (after-tax)	0.0	0.0	0.0	0.0
Non-core income adjustments (after-tax)	2.8		11.1
Change in MRB (after-tax)		0.0		0.0
Intangible asset amortization (after-tax)		(2.8)		(8.5)
Net income	15.0	14.8	34.1	39.9
Corporate & Other
Segment Reporting Information [Line Items]
Net premiums and contract charges earned	0.0	0.0	0.0	0.0
Net investment income	(0.3)	(0.5)	3.8	(1.5)
Other segment income	0.5	0.7	1.5	2.0
Total segment revenues	0.2	0.2	5.3	0.5
Benefits and claims expenses (excluding catastrophe losses)	0.0	0.0	0.0	0.0
Catastrophe losses	0.0	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0	0.0
Interest credited	0.0	0.0	0.0	0.0
Operating & admin expenses	2.8	2.7	8.8	7.7
Commissions expense	0.0	0.0	0.0	0.0
Taxes, licenses and fees	0.2	0.1	0.5	0.6
Deferred policy acquisition costs	0.0	0.0	0.0	0.0
Deferred policy acquisition cost amortization	0.0	0.0	0.0	0.0
Interest expense	8.9	8.7	26.4	26.1
Total segment expenses	11.9	11.5	35.7	34.4
Pretax profit (loss)	(11.7)	(11.3)	(30.4)	(33.9)
Income tax expense	(3.6)	(2.3)	(8.0)	(7.0)
Segment profit (loss) (Core earnings)	(8.1)	(9.0)	(22.4)	(26.9)
Net investment losses (after-tax)	(2.7)	3.0	4.6	0.1
Non-core income adjustments (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	$ (5.4)	$ (6.0)	$ (27.0)	$ (26.8)