Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 268.9	$ 212.8	$ 700.7	$ 558.7
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	$ 60.8	$ 52.7	$ 171.4	$ 144.2
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.60%	24.80%	24.50%	25.80%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 38.2	$ 33.7	$ 104.1	$ 91.2
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	14.20%	15.80%	14.90%	16.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 28.6	$ 23.1	$ 72.4	$ 61.4
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.60%	10.90%	10.30%	11.00%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 13.2	$ 10.9	$ 33.8	$ 27.0
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.90%	5.10%	4.80%	4.80%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 13.0	$ 10.7	$ 33.4	$ 28.5
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.80%	5.00%	4.80%	5.10%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 11.0	$ 7.2	$ 28.1	$ 19.4
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.10%	3.40%	4.00%	3.50%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.7	$ 6.6	$ 22.8	$ 16.4
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.20%	3.10%	3.30%	2.90%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.0	$ 6.6	$ 20.4	$ 17.0
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	3.10%	2.90%	3.00%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 7.6	$ 6.2	$ 18.8	$ 14.9
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.90%	2.70%	2.70%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.3	$ 4.8	$ 17.0	$ 13.5
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.30%	2.30%	2.40%	2.40%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 73.5	$ 50.3	$ 178.5	$ 125.2
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	27.50%	23.60%	25.40%	22.50%