Segment Information - Gross Written Premium by State (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1,171.3	$ 929.0	$ 738.4
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 256.8	$ 225.6	$ 191.6
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.90%	24.30%	25.90%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 148.8	$ 133.7	$ 117.5
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.70%	14.40%	15.90%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 116.4	$ 96.3	$ 80.8
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	9.90%	10.40%	10.90%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 60.2	$ 38.9	$ 25.9
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.20%	3.50%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 54.0	$ 43.7	$ 35.3
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.60%	4.70%	4.80%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 52.7	$ 44.3	$ 37.6
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.50%	4.80%	5.10%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 40.9	$ 29.2	$ 22.0
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.50%	3.10%	3.00%
Florida
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 33.8	$ 19.4	$ 9.3
Florida | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	2.10%	1.30%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 33.0	$ 24.9	$ 19.7
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.70%	2.70%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 29.4	$ 22.4	$ 18.0
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	2.40%	2.40%
All others
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 299.0	$ 230.6	$ 171.4
All others | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.60%	24.70%	23.20%
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1,125.0	$ 909.0	$ 729.1
United States | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	96.00%	97.80%	98.70%
Europe and UK
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 46.3	$ 20.0	$ 9.3
Europe and UK | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.00%	2.20%	1.30%