Short-Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 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	$ 596.6		596.6
Property & Casualty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	418.2	$ 397.8	416.8	$ 388.7
Less: reinsurance recoverables	103.6	101.3	104.0	100.8
Net reserves, beginning of year	314.6	296.5	312.8	287.9
Incurred claims and claim expenses:
Claims occurring in the current year	159.0	152.0	283.7	280.8
Decrease in estimated reserves for claims occurring in prior years	(6.2)	0.0	(6.2)	0.0
Total claims and claim expenses incurred	152.8	152.0	277.5	280.8
Claims and claim expense payments for claims occurring during:
Current year	94.2	90.2	138.7	130.8
Prior years	42.0	46.8	120.4	126.4
Total claims and claim expense payments	136.2	137.0	259.1	257.2
Net reserves, end of year	331.2	311.5	331.2	311.5
Plus: reinsurance recoverables	101.7	104.3	101.7	104.3
Gross reserves, end of year	432.9	415.8	432.9	415.8
Group Benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	114.3	120.3	116.6	121.6
Less: reinsurance recoverables	27.6	28.0	27.7	27.9
Net reserves, beginning of year	86.7	92.3	88.9	93.7
Incurred claims and claim expenses:
Claims occurring in the current year	19.4	20.8	39.6	41.0
Decrease in estimated reserves for claims occurring in prior years	(3.3)	(3.0)	(9.4)	(7.9)
Total claims and claim expenses incurred	16.1	17.8	30.2	33.1
Claims and claim expense payments for claims occurring during:
Current year	8.2	8.9	12.1	13.1
Prior years	6.5	7.6	18.9	20.0
Total claims and claim expense payments	14.7	16.5	31.0	33.1
Net reserves, end of year	88.1	93.7	88.1	93.7
Plus: reinsurance recoverables	28.0	28.2	28.0	28.2
Gross reserves, end of year	$ 116.1	$ 121.9	$ 116.1	$ 121.9