Segment Information - Schedule of Gross Written Premium by State (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 333.3	$ 268.9	$ 872.0	$ 700.7
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	$ 70.2	$ 60.8	$ 191.9	$ 171.4
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.10%	22.60%	22.00%	24.50%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 42.0	$ 38.2	$ 113.3	$ 104.1
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.60%	14.20%	13.00%	14.90%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 34.1	$ 28.6	$ 87.0	$ 72.4
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.20%	10.60%	10.00%	10.30%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 17.0	$ 11.0	$ 44.5	$ 28.1
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.10%	5.10%	4.00%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 16.0	$ 13.2	$ 41.7	$ 33.8
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%	4.80%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 14.9	$ 13.0	$ 39.4	$ 33.4
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.50%	4.80%	4.50%	4.80%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 11.8	$ 8.7	$ 29.3	$ 22.8
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.50%	3.20%	3.40%	3.30%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.5	$ 7.6	$ 24.8	$ 18.8
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	2.80%	2.80%	2.70%
Florida
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.8	$ 6.1	$ 24.3	$ 13.5
Florida | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	2.30%	2.80%	1.90%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.2	$ 8.0	$ 22.1	$ 20.4
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	3.00%	2.50%	2.90%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 86.5	$ 67.0	$ 221.4	$ 168.8
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	26.00%	24.90%	25.40%	24.00%
US
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 320.0	$ 262.2	$ 839.7	$ 687.5
US | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	96.10%	97.50%	96.30%	98.10%
Europe and UK
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 13.3	$ 6.7	$ 32.3	$ 13.2
Europe and UK | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.90%	2.50%	3.70%	1.90%