Short-Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information
Beginning gross reserves			$ 564.0
Claims and claim expense payments for claims occurring during:
Ending gross reserves	$ 586.6		586.6
Property & Casualty
Segment Reporting Information
Beginning gross reserves	415.8	$ 378.4	388.7	$ 362.4
Less: reinsurance recoverables	104.3	109.8	100.8	110.3
Net reserves, beginning of period	311.5	268.6	287.9	252.1
Incurred claims and claim expenses:
Claims occurring in the current period	149.2	120.3	430.0	372.8
Increase (decrease) in estimated reserves for claims occurring in prior periods	0.0	2.0	0.0	8.0
Total claims and claim expenses incurred	149.2	122.3	430.0	380.8
Claims and claim expense payments for claims occurring during:
Current period	115.9	102.8	246.7	221.1
Prior periods	30.9	29.7	157.3	153.4
Total claims and claim expense payments	146.8	132.5	404.0	374.5
Net reserves, end of period	313.9	258.4	313.9	258.4
Plus: reinsurance recoverables	105.4	109.0	105.4	109.0
Ending gross reserves	419.3	367.4	419.3	367.4
Group Benefits
Segment Reporting Information
Beginning gross reserves	121.9	130.2	121.6	125.4
Less: reinsurance recoverables	28.2	29.6	27.9	28.6
Net reserves, beginning of period	93.7	100.6	93.7	96.8
Incurred claims and claim expenses:
Claims occurring in the current period	20.0	20.7	61.0	65.6
Increase (decrease) in estimated reserves for claims occurring in prior periods	(7.6)	(9.7)	(15.5)	(11.7)
Total claims and claim expenses incurred	12.4	11.0	45.5	53.9
Claims and claim expense payments for claims occurring during:
Current period	12.2	10.4	25.3	26.2
Prior periods	4.3	5.4	24.3	28.7
Total claims and claim expense payments	16.5	15.8	49.6	54.9
Net reserves, end of period	89.6	95.8	89.6	95.8
Plus: reinsurance recoverables	27.5	28.0	27.5	28.0
Ending gross reserves	$ 117.1	$ 123.8	$ 117.1	$ 123.8