Short-Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information
Beginning gross reserves			$ 564.0
Claims and claim expense payments for claims occurring during:
Ending gross reserves	$ 597.8		597.8
Property & Casualty
Segment Reporting Information
Beginning gross reserves	397.8	$ 356.2	388.7	$ 362.4
Less: reinsurance recoverables	101.3	108.0	100.8	110.3
Net reserves, beginning of period	296.5	248.2	287.9	252.1
Incurred claims and claim expenses:
Claims occurring in the current period	152.0	144.1	280.8	252.4
Increase (decrease) in estimated reserves for claims occurring in prior periods	0.0	6.0	0.0	6.0
Total claims and claim expenses incurred	152.0	150.1	280.8	258.4
Claims and claim expense payments for claims occurring during:
Current period	90.2	84.4	130.8	118.2
Prior periods	46.8	45.3	126.4	123.7
Total claims and claim expense payments	137.0	129.7	257.2	241.9
Net reserves, end of period	311.5	268.6	311.5	268.6
Plus: reinsurance recoverables	104.3	109.8	104.3	109.8
Ending gross reserves	415.8	378.4	415.8	378.4
Group Benefits
Segment Reporting Information
Beginning gross reserves	120.3	133.2	121.6	125.4
Less: reinsurance recoverables	28.0	29.8	27.9	28.6
Net reserves, beginning of period	92.3	103.4	93.7	96.8
Incurred claims and claim expenses:
Claims occurring in the current period	20.8	16.2	41.0	44.9
Increase (decrease) in estimated reserves for claims occurring in prior periods	(3.0)	(0.1)	(7.9)	(2.0)
Total claims and claim expenses incurred	17.8	16.1	33.1	42.9
Claims and claim expense payments for claims occurring during:
Current period	8.9	11.4	13.1	15.8
Prior periods	7.6	7.5	20.0	23.4
Total claims and claim expense payments	16.5	18.9	33.1	39.2
Net reserves, end of period	93.7	100.6	93.7	100.6
Plus: reinsurance recoverables	28.2	29.6	28.2	29.6
Ending gross reserves	$ 121.9	$ 130.2	$ 121.9	$ 130.2