Unpaid Claims and Claim Expenses - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Beginning gross reserves	$ 438.8
Claims and claim expense payments for claims occurring during:
Ending gross reserves	452.5
Benefits, claims and settlement expenses	134.3	$ 138.7
Property and Casualty
Segment Reporting Information [Line Items]
Beginning gross reserves	372.2	387.0
Less: reinsurance recoverables	112.9	120.5
Net reserves, beginning of period	259.3	266.5
Incurred claims and claim expenses:
Claims occurring in the current period	94.8	105.5
Decrease in estimated reserves for claims occurring in prior periods	0.0	(1.0)
Total claims and claim expenses incurred	94.8	104.5
Claims and claim expense payments for claims occurring during:
Current period	34.4	43.5
Prior periods	58.1	64.3
Total claims and claim expense payments	92.5	107.8
Ending gross reserves	374.1	382.2
Plus: reinsurance recoverables	112.5	119.0
Net reserves, end of period	261.6	263.2
Life and Annuity Segments
Claims and claim expense payments for claims occurring during:
Net reserves, end of period	78.4	56.2
Benefits, claims and settlement expenses	$ 39.5	$ 34.2