Segment Information - Schedule of Financial Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues
Net premiums and contract charges earned	$ 1,146.0	$ 1,057.1	$ 1,027.7	[1]
Net investment income	445.7	444.8	400.9	[1]
Other segment income	20.8	14.0	9.5
Total segment revenues	1,612.5	1,515.9	1,438.1
Benefits, losses and expenses
Benefits and claims expenses (excluding catastrophe losses)	557.3	600.0	600.7
Catastrophe losses	94.9	97.6	80.0
Loss adjustment expenses	72.8	71.5	66.3
Interest credited	215.9	205.7	173.4	[1]
Operating & admin expenses	294.9	267.0	255.8
Commissions expense	142.2	130.4	125.1
Taxes, licenses and fees	30.6	27.7	29.0
Deferred policy acquisition costs	(122.2)	(107.0)	(94.4)
Deferred policy acquisition cost amortization	111.1	101.2	88.2	[1]
Intangible asset amortization	14.5	14.8	16.8	[1]
Interest expense	34.6	29.7	19.4	[1]
Total segment expenses	1,446.6	1,438.6	1,360.3
Pretax profit (loss)	165.9	77.3	77.8
Income tax expense	33.8	13.5	9.7
Segment profit (loss) (Core earnings)	132.1	63.8	68.1
Net investment losses (after-tax)	(13.6)	(18.8)	(44.5)
Non-core Legacy Commercial exposures (before-tax)	(20.0)
Non-core Legacy Commercial exposures (after-tax)	(15.7)
Goodwill and intangible asset impairments (after-tax)			(3.8)
Net income	102.8	45.0	19.8	[1]
Property & Casualty
Revenues
Net premiums and contract charges earned	736.5	645.6	608.2
Net investment income	46.0	37.9	31.4
Other segment income	2.3	2.8	3.4
Total segment revenues	784.8	686.3	643.0
Benefits, losses and expenses
Benefits and claims expenses (excluding catastrophe losses)	353.3	387.9	388.0
Catastrophe losses	94.9	97.6	80.0
Loss adjustment expenses	72.8	71.5	66.3
Interest credited	0.0	0.0	0.0
Operating & admin expenses	122.4	109.2	104.9
Commissions expense	63.0	52.6	43.8
Taxes, licenses and fees	20.2	17.7	18.4
Deferred policy acquisition costs	(89.8)	(76.2)	(64.5)
Deferred policy acquisition cost amortization	84.6	71.3	64.3
Intangible asset amortization	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0
Total segment expenses	721.4	731.6	701.2
Pretax profit (loss)	63.4	(45.3)	(58.2)
Income tax expense	14.3	(9.8)	(13.8)
Segment profit (loss) (Core earnings)	49.1	(35.5)	(44.4)
Net investment losses (after-tax)	0.0	0.0	0.0
Non-core Legacy Commercial exposures (before-tax)	0.0
Non-core Legacy Commercial exposures (after-tax)	0.0
Goodwill and intangible asset impairments (after-tax)			0.0
Net income	49.1	(35.5)	(44.4)
Life & Retirement
Revenues
Net premiums and contract charges earned	154.6	151.7	144.0
Net investment income	363.6	369.9	338.3
Other segment income	20.2	17.0	17.0
Total segment revenues	538.4	538.6	499.3
Benefits, losses and expenses
Benefits and claims expenses (excluding catastrophe losses)	125.2	123.2	121.5
Catastrophe losses	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0
Interest credited	211.2	201.8	172.1
Operating & admin expenses	93.2	83.9	88.5
Commissions expense	41.1	37.2	37.2
Taxes, licenses and fees	3.8	3.9	4.1
Deferred policy acquisition costs	(28.3)	(26.2)	(27.4)
Deferred policy acquisition cost amortization	24.6	28.0	23.0
Intangible asset amortization	0.2	0.2	1.1
Interest expense	0.0	0.0	0.0
Total segment expenses	471.0	452.0	420.1
Pretax profit (loss)	67.4	86.6	79.2
Income tax expense	11.1	15.1	11.6
Segment profit (loss) (Core earnings)	56.3	71.5	67.6
Net investment losses (after-tax)	0.0	0.0	0.0
Non-core Legacy Commercial exposures (before-tax)	0.0
Non-core Legacy Commercial exposures (after-tax)	0.0
Goodwill and intangible asset impairments (after-tax)			(3.8)
Net income	56.3	71.5	63.8
Supplemental & Group Benefits
Revenues
Net premiums and contract charges earned	254.9	259.8	275.5
Net investment income	38.1	38.9	33.3
Other segment income	(4.6)	(11.1)	(13.4)
Total segment revenues	288.4	287.6	295.4
Benefits, losses and expenses
Benefits and claims expenses (excluding catastrophe losses)	78.8	88.9	91.2
Catastrophe losses	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0
Interest credited	4.7	3.9	1.3
Operating & admin expenses	70.7	66.9	54.6
Commissions expense	38.1	40.6	44.1
Taxes, licenses and fees	5.9	5.7	6.1
Deferred policy acquisition costs	(4.1)	(4.6)	(2.5)
Deferred policy acquisition cost amortization	1.9	1.9	0.9
Intangible asset amortization	14.3	14.6	15.7
Interest expense	0.0	0.0	0.0
Total segment expenses	210.3	217.9	211.4
Pretax profit (loss)	78.1	69.7	84.0
Income tax expense	17.7	14.8	18.1
Segment profit (loss) (Core earnings)	60.4	54.9	65.9
Net investment losses (after-tax)	0.0	0.0	0.0
Non-core Legacy Commercial exposures (before-tax)	0.0
Non-core Legacy Commercial exposures (after-tax)	0.0
Goodwill and intangible asset impairments (after-tax)			0.0
Net income	60.4	54.9	65.9
Corporate & Other
Revenues
Net premiums and contract charges earned	0.0	0.0	0.0
Net investment income	(2.0)	(1.9)	(2.1)
Other segment income	2.9	5.3	2.5
Total segment revenues	0.9	3.4	0.4
Benefits, losses and expenses
Benefits and claims expenses (excluding catastrophe losses)	0.0	0.0	0.0
Catastrophe losses	0.0	0.0	0.0
Loss adjustment expenses	0.0	0.0	0.0
Interest credited	0.0	0.0	0.0
Operating & admin expenses	8.6	7.0	7.8
Commissions expense	0.0	0.0	0.0
Taxes, licenses and fees	0.7	0.4	0.4
Deferred policy acquisition costs	0.0	0.0	0.0
Deferred policy acquisition cost amortization	0.0	0.0	0.0
Intangible asset amortization	0.0	0.0	0.0
Interest expense	34.6	29.7	19.4
Total segment expenses	43.9	37.1	27.6
Pretax profit (loss)	(43.0)	(33.7)	(27.2)
Income tax expense	(9.3)	(6.6)	(6.2)
Segment profit (loss) (Core earnings)	(33.7)	(27.1)	(21.0)
Net investment losses (after-tax)	(13.6)	(18.8)	(44.5)
Non-core Legacy Commercial exposures (before-tax)	(20.0)
Non-core Legacy Commercial exposures (after-tax)	(15.7)
Goodwill and intangible asset impairments (after-tax)			0.0
Net income	$ (63.0)	$ (45.9)	$ (65.5)

[1] Recast for the adoption of ASU 2018-12.