Short-Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 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	584.4
Group Benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	116.6	$ 121.6
Less: reinsurance recoverables	27.7	27.9
Net reserves, beginning of year	88.9	93.7
Incurred claims and claim expenses:
Claims occurring in the current year	20.2	20.2
Decrease in estimated reserves for claims occurring in prior years	(6.1)	(4.9)
Total claims and claim expenses incurred	14.1	15.3
Claims and claim expense payments for claims occurring during:
Current year	3.9	4.2
Prior years	12.4	12.4
Total claims and claim expense payments	16.3	16.6
Net reserves, end of year	86.7	92.3
Plus: reinsurance recoverables	27.6	28.0
Gross reserves, end of year	114.3	120.3
Property & Casualty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	416.8	388.7
Less: reinsurance recoverables	104.0	100.8
Net reserves, beginning of year	312.8	287.9
Incurred claims and claim expenses:
Claims occurring in the current year	124.7	128.8
Decrease in estimated reserves for claims occurring in prior years	0.0	0.0
Total claims and claim expenses incurred	124.7	128.8
Claims and claim expense payments for claims occurring during:
Current year	44.6	40.6
Prior years	78.3	79.6
Total claims and claim expense payments	122.9	120.2
Net reserves, end of year	314.6	296.5
Plus: reinsurance recoverables	103.6	101.3
Gross reserves, end of year	$ 418.2	$ 397.8