Unpaid Loss and Loss Adjustment Expense - Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid loss and LAE as of January 1	$ 256.2	$ 97.9
Less: Reinsurance recoverable	124.6	72.7
Net unpaid loss and LAE as of January 1	131.6	25.2
Add: Incurred losses and LAE, net of reinsurance, related to:
Current year	286.2	170.5
Prior years	(5.8)	(3.2)
Total incurred	280.4	167.3	$ 71.9
Deduct: Paid losses and LAE, net of reinsurance, related to:
Current year	189.4	106.9
Prior years	80.5	30.1
Total paid	269.9	137.0
Unpaid loss and LAE, net of reinsurance recoverable, as of December 31	142.1	131.6	25.2
Reinsurance recoverable at December 31	120.2	124.6	72.7
Unpaid loss and LAE, gross of reinsurance recoverable, as of December 31	262.3	256.2	$ 97.9
Metromile
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net unpaid loss and LAE as of January 1	76.2
Deduct: Paid losses and LAE, net of reinsurance, related to:
Unpaid loss and LAE, net of reinsurance recoverable, as of December 31	$ 0.0	$ 76.2