Unpaid Claims and Claim Expenses - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Beginning gross reserves			$ 438.8
Claims and claim expense payments for claims occurring during:
Ending gross reserves	$ 433.6		433.6
Benefits, claims and settlement expenses	147.1	$ 143.0	281.4	$ 281.7
Property and Casualty
Segment Reporting Information [Line Items]
Beginning gross reserves	374.1	382.2	372.2	387.0
Less: reinsurance recoverables	112.5	119.0	112.9	120.5
Net reserves, beginning of period	261.6	263.2	259.3	266.5
Incurred claims and claim expenses:
Claims occurring in the current period	118.1	109.1	212.9	214.6
Decrease in estimated reserves for claims occurring in prior periods	(4.2)	(1.0)	(4.2)	(2.0)
Total claims and claim expenses incurred	113.9	108.1	208.7	212.6
Claims and claim expense payments for claims occurring during:
Current period	79.1	61.4	113.5	104.9
Prior periods	36.9	37.5	95.0	101.8
Total claims and claim expense payments	116.0	98.9	208.5	206.7
Ending gross reserves	368.4	388.5	368.4	388.5
Plus: reinsurance recoverables	108.9	116.1	108.9	116.1
Net reserves, end of period	259.5	272.4	259.5	272.4
Life and Annuity Segments
Claims and claim expense payments for claims occurring during:
Net reserves, end of period	65.2	56.1	65.2	56.1
Benefits, claims and settlement expenses	$ 33.2	$ 34.9	$ 72.7	$ 69.1