Geographical Breakdown of Gross Written Premium (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 738.4	$ 555.7	$ 375.7
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 191.6	$ 142.0	$ 93.9
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.90%	25.60%	25.00%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 117.5	$ 91.3	$ 72.5
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	15.90%	16.40%	19.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 80.8	$ 66.0	$ 47.3
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.90%	11.90%	12.60%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 37.6	$ 28.3	$ 16.7
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	5.10%	4.40%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 35.3	$ 26.3	$ 15.7
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.80%	4.70%	4.20%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 25.9	$ 15.8	$ 5.4
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.50%	2.80%	1.40%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 22.6	$ 19.8	$ 16.5
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.10%	3.60%	4.40%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 22.0	$ 15.8	$ 9.2
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	2.80%	2.40%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 19.7	$ 14.4	$ 9.7
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%
Oregon
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 18.8	$ 13.0	$ 6.9
Oregon | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	2.30%	1.80%
All others
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 166.6	$ 123.0	$ 81.9
All others | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.60%	22.20%	21.80%
Total US | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%