Short-Term Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information
Beginning gross reserves	$ 564.0
Claims and claim expense payments for claims occurring during:
Ending gross reserves	574.8
Property & Casualty
Segment Reporting Information
Beginning gross reserves	388.7	$ 362.4
Less: reinsurance recoverables	100.8	110.3
Net reserves, beginning of period	287.9	252.1
Incurred claims and claim expenses:
Claims occurring in the current period	128.8	108.3
Decrease in estimated reserves for claims occurring in prior periods	0.0	0.0
Total claims and claim expenses incurred	128.8	108.3
Claims and claim expense payments for claims occurring during:
Current period	40.6	33.8
Prior periods	79.6	78.4
Total claims and claim expense payments	120.2	112.2
Net reserves, end of period	296.5	248.2
Plus: reinsurance recoverables	101.3	108.0
Ending gross reserves	397.8	356.2
Group Benefits
Segment Reporting Information
Beginning gross reserves	121.6	125.4
Less: reinsurance recoverables	27.9	28.6
Net reserves, beginning of period	93.7	96.8
Incurred claims and claim expenses:
Claims occurring in the current period	20.2	28.8
Decrease in estimated reserves for claims occurring in prior periods	(4.9)	(1.9)
Total claims and claim expenses incurred	15.3	26.9
Claims and claim expense payments for claims occurring during:
Current period	4.2	4.4
Prior periods	12.4	15.9
Total claims and claim expense payments	16.7	20.3
Net reserves, end of period	92.3	103.4
Plus: reinsurance recoverables	28.0	29.8
Ending gross reserves	$ 120.3	$ 133.2