Unpaid Loss and Loss Adjustment Expense - Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid loss and LAE at beginning of period	$ 28.2	$ 13.1
Less: Reinsurance recoverable at beginning of period	18.5	11.3
Net unpaid loss and LAE at beginning of period	9.7	1.8
Add: Incurred losses and LAE, net of reinsurance, related to:
Current year	54.8	47.3
Prior years	(0.1)	(1.5)
Total incurred	54.7	45.8	$ 15.2
Deduct: Paid losses and LAE, net of reinsurance, related to:
Current year	44.6	37.7
Prior years	9.8	0.2
Total paid	54.4	37.9
Unpaid loss and LAE, net of reinsurance recoverable, at end of period	10.0	9.7	1.8
Reinsurance recoverable at end of period	36.3	18.5	11.3
Unpaid loss and LAE, gross of reinsurance recoverable, at end of period	$ 46.3	$ 28.2	$ 13.1