Unpaid Loss and Loss Adjustment Expense - Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid loss and LAE at beginning of period			$ 97.9	$ 46.3
Less: Reinsurance recoverable at beginning of period			72.7	36.3
Net unpaid loss and LAE at beginning of period			25.2	10.0
Add: Incurred loss and LAE, net of reinsurance, related to:
Current year			106.9	47.3
Prior years			(1.1)	(0.3)
Total incurred	$ 53.3	$ 17.5	105.8	47.0
Deduct: Paid loss and LAE, net of reinsurance, related to:
Current year			65.9	32.3
Prior years			21.9	6.3
Total paid			87.8	38.6
Unpaid loss and LAE, net of reinsurance recoverable acquired from Metromile	111.3	18.4	111.3	18.4
Unpaid loss and LAE, net of reinsurance recoverable, at end of period	111.3	18.4	111.3	18.4
Reinsurance recoverable at end of period	109.3	55.6	109.3	55.6
Unpaid loss and LAE, gross of reinsurance recoverable, at end of period	220.6	74.0	220.6	74.0
Metromile
Deduct: Paid loss and LAE, net of reinsurance, related to:
Unpaid loss and LAE, net of reinsurance recoverable acquired from Metromile	68.1	0.0	68.1	0.0
Unpaid loss and LAE, net of reinsurance recoverable, at end of period	$ 68.1	$ 0.0	$ 68.1	$ 0.0