Short-Duration Insurance Contracts - Schedule of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year			$ 581.7
Claims and claim expense payments for claims occurring during:
Gross reserves, end of year	$ 597.0		597.0
Property & Casualty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	432.9	$ 415.8	416.8	$ 388.7
Less: reinsurance recoverables	101.7	104.3	104.0	100.8
Net reserves, beginning of year	331.2	311.5	312.8	287.9
Incurred claims and claim expenses:
Claims occurring in the current year	147.8	149.2	431.4	430.0
Decrease in estimated reserves for claims occurring in prior years	(13.0)	0.0	(19.2)	0.0
Total claims and claim expenses incurred	134.8	149.2	412.2	430.0
Claims and claim expense payments for claims occurring during:
Current year	102.3	115.9	241.0	246.7
Prior years	31.6	30.9	152.0	157.3
Total claims and claim expense payments	133.9	146.8	393.0	404.0
Net reserves, end of year	332.1	313.9	332.1	313.9
Plus: reinsurance recoverables	101.3	105.4	101.3	105.4
Gross reserves, end of year	433.4	419.3	433.4	419.3
Group Benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	116.1	121.9	116.6	121.6
Less: reinsurance recoverables	28.0	28.2	27.7	27.9
Net reserves, beginning of year	88.1	93.7	88.9	93.7
Incurred claims and claim expenses:
Claims occurring in the current year	18.5	20.0	58.0	61.0
Decrease in estimated reserves for claims occurring in prior years	(3.9)	(7.6)	(13.2)	(15.5)
Total claims and claim expenses incurred	14.6	12.4	44.8	45.5
Claims and claim expense payments for claims occurring during:
Current year	10.5	12.2	22.6	25.3
Prior years	4.8	4.3	23.7	24.3
Total claims and claim expense payments	15.3	16.5	46.3	49.6
Net reserves, end of year	87.4	89.6	87.4	89.6
Plus: reinsurance recoverables	27.8	27.5	27.8	27.5
Gross reserves, end of year	$ 115.2	$ 117.1	$ 115.2	$ 117.1