Unpaid Claims and Claim Expenses - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information
Beginning gross reserves			$ 438.8
Claims and claim expense payments for claims occurring during:
Ending gross reserves	$ 440.1		440.1
Benefits, claims and settlement expenses	164.8	$ 151.4	446.2	$ 433.1
Property and Casualty
Segment Reporting Information
Beginning gross reserves	368.4	388.5	372.2	387.0
Less: reinsurance recoverables	108.9	116.1	112.9	120.5
Net reserves, beginning of period	259.5	272.4	259.3	266.5
Incurred claims and claim expenses:
Claims occurring in the current period	132.5	125.6	345.4	340.2
Decrease in estimated reserves for claims occurring in prior periods	(3.0)	(7.2)	(7.2)	(9.2)
Total claims and claim expenses incurred	129.5	118.4	338.2	331.0
Claims and claim expense payments for claims occurring during:
Current period	96.8	91.0	210.3	195.9
Prior periods	26.4	19.3	121.4	121.1
Total claims and claim expense payments	123.2	110.3	331.7	317.0
Net reserves, end of period	265.8	280.5	265.8	280.5
Plus: reinsurance recoverables	109.6	114.8	109.6	114.8
Ending gross reserves	375.4	395.3	375.4	395.3
Life and Annuity Segments
Claims and claim expense payments for claims occurring during:
Net reserves, end of period	64.7	59.2	64.7	59.2
Benefits, claims and settlement expenses	$ 35.3	$ 33.0	$ 108.0	$ 102.1