Liability for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Losses and loss adjustment expenses	$ 315.2	$ 302.4	$ 365.9
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and LAE, gross of reinsurance, at beginning of period	2,069.4	2,192.8	2,207.9
Loss reinsurance recoverable, excluding bad debt, on unpaid losses	497.0	532.5	504.4
Net unpaid losses and LAE at beginning of period	1,572.4	1,660.3	1,703.5
Losses and LAE, net of reinsurance, incurred in:
Current year	366.5	395.9	456.1
Prior years	(39.8)	(81.6)	(77.5)
Total net losses and LAE incurred during the period	326.7	314.3	378.6
Deduct payments for losses and LAE, net of reinsurance, related to:
Current year	76.6	83.6	106.6
Prior years	318.2	318.6	315.2
Total net payments for losses and LAE during the period	394.8	402.2	421.8
Ending unpaid losses and LAE, net of reinsurance	1,504.3	1,572.4	1,660.3
Loss reinsurance recoverable, excluding bad debt, on unpaid losses	476.9	497.0	532.5
Unpaid losses and LAE, gross of reinsurance, at end of period	1,981.2	2,069.4	$ 2,192.8
Salvage and subrogation recoveries, value	105.6	28.1
Accident Year 2020
Losses and LAE, net of reinsurance, incurred in:
Prior years	8.0
Accident Year 2019
Losses and LAE, net of reinsurance, incurred in:
Prior years	10.0	13.3
Voluntary risk business
Losses and LAE, net of reinsurance, incurred in:
Prior years	(38.0)	(80.2)
Involuntary assigned risk business
Losses and LAE, net of reinsurance, incurred in:
Prior years	$ (1.8)	$ (1.4)