Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 212.8	$ 174.0	$ 558.7	$ 410.7
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	$ 52.7	$ 45.9	$ 144.2	$ 101.3
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.80%	26.40%	25.80%	24.70%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 33.7	$ 26.6	$ 91.2	$ 70.9
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	15.80%	15.30%	16.30%	17.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 23.1	$ 19.1	$ 61.4	$ 49.8
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.90%	11.00%	11.00%	12.10%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.7	$ 8.9	$ 28.5	$ 20.1
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.00%	5.10%	5.10%	4.90%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.9	$ 8.8	$ 27.0	$ 19.8
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	5.10%	4.80%	4.80%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 7.2	$ 5.6	$ 19.4	$ 10.0
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.40%	3.20%	3.50%	2.40%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.6	$ 6.1	$ 17.0	$ 15.5
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.10%	3.50%	3.00%	3.80%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.6	$ 4.9	$ 16.4	$ 11.8
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.10%	2.80%	2.90%	2.90%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.2	$ 4.8	$ 14.9	$ 10.7
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	2.80%	2.70%	2.60%
Oregon
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.0	$ 4.1	$ 14.3	$ 8.5
Oregon | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.30%	2.40%	2.60%	2.10%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 50.1	$ 39.2	$ 124.4	$ 92.3
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	23.60%	22.40%	22.30%	22.40%