Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 181.9	$ 126.1	$ 345.9	$ 236.7
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	$ 46.0	$ 28.3	$ 91.5	$ 55.4
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.30%	22.40%	26.50%	23.40%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 31.4	$ 24.1	$ 57.5	$ 44.3
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	17.30%	19.10%	16.60%	18.70%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 19.9	$ 15.9	$ 38.3	$ 30.7
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.90%	12.60%	11.10%	13.00%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.2	$ 5.9	$ 17.8	$ 11.2
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.70%	5.10%	4.70%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.0	$ 6.6	$ 16.1	$ 11.0
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.90%	5.20%	4.70%	4.60%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.1	$ 2.4	$ 12.2	$ 4.4
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.40%	1.90%	3.50%	1.90%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.5	$ 5.0	$ 10.4	$ 9.4
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	4.00%	3.00%	4.00%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.5	$ 3.9	$ 9.8	$ 6.9
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	3.10%	2.80%	2.90%
Oregon
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.8	$ 2.4	$ 9.3	$ 4.4
Oregon | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	1.90%	2.70%	1.90%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.7	$ 3.2	$ 8.7	$ 5.9
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	2.50%	2.50%	2.50%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 39.8	$ 28.4	$ 74.3	$ 53.1
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.90%	22.60%	21.50%	22.40%