Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 174.0	$ 116.8	$ 410.7	$ 282.1
Number of reportable segments | segment			1
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 45.9	$ 29.0	$ 101.3	$ 70.4
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	26.40%	24.80%	24.70%	25.00%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 26.6	$ 21.7	$ 70.9	$ 55.4
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	15.30%	18.60%	17.30%	19.60%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 19.1	$ 14.3	$ 49.8	$ 35.2
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	11.00%	12.20%	12.10%	12.50%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.9	$ 5.3	$ 20.1	$ 12.1
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.50%	4.90%	4.30%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.8	$ 5.0	$ 19.8	$ 12.1
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.30%	4.80%	4.30%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.1	$ 4.9	$ 15.5	$ 12.9
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.50%	4.20%	3.80%	4.60%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.9	$ 3.1	$ 11.8	$ 6.5
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.70%	2.90%	2.30%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.8	$ 3.3	$ 10.7	$ 7.4
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.80%	2.60%	2.60%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.6	$ 1.8	$ 10.0	$ 3.9
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.20%	1.50%	2.40%	1.40%
Virginia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.4	$ 2.6	$ 9.5	$ 6.0
Virginia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	2.20%	2.30%	2.10%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 38.9	$ 25.8	$ 91.3	$ 60.2
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.30%	22.20%	22.20%	21.30%