Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) segment	Jun. 30, 2023 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 226.2	$ 181.9	$ 431.8	$ 345.9
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	$ 55.3	$ 46.0	$ 110.6	$ 91.5
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.40%	25.30%	25.60%	26.50%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 35.9	$ 31.4	$ 65.9	$ 57.5
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	15.90%	17.30%	15.30%	16.60%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 22.8	$ 19.9	$ 43.8	$ 38.3
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.10%	10.90%	10.10%	11.10%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.9	$ 9.0	$ 20.6	$ 16.1
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.80%	4.90%	4.80%	4.70%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.7	$ 9.2	$ 20.4	$ 17.8
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.70%	5.10%	4.70%	5.10%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.1	$ 6.1	$ 17.1	$ 12.2
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.00%	3.40%	4.00%	3.50%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.2	$ 5.5	$ 14.1	$ 9.8
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.60%	3.00%	3.30%	2.80%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.8	$ 5.5	$ 12.4	$ 10.4
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	3.00%	2.90%	3.00%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.1	$ 4.7	$ 11.2	$ 8.7
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.70%	2.60%	2.60%	2.50%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.7	$ 4.6	$ 10.7	$ 8.7
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	2.60%	2.50%	2.50%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 54.7	$ 40.0	$ 105.0	$ 74.9
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.30%	21.90%	24.20%	21.70%