Segment Information - Schedule of Gross Written Premium by State (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 284.5	$ 226.2	$ 538.7	$ 431.8
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	$ 61.3	$ 55.3	$ 121.7	$ 110.6
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.50%	24.40%	22.60%	25.60%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 38.8	$ 35.9	$ 71.3	$ 65.9
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	13.60%	15.90%	13.20%	15.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 27.0	$ 22.8	$ 52.9	$ 43.8
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	9.50%	10.10%	9.80%	10.10%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 14.5	$ 9.1	$ 27.5	$ 17.1
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.10%	4.00%	5.10%	4.00%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 14.1	$ 10.9	$ 25.7	$ 20.6
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.00%	4.80%	4.80%	4.80%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 12.8	$ 10.7	$ 24.5	$ 20.4
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.50%	4.70%	4.50%	4.70%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.7	$ 8.2	$ 17.5	$ 14.1
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.80%	3.60%	3.20%	3.30%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.3	$ 6.1	$ 15.3	$ 11.2
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	2.70%	2.80%	2.60%
Florida
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.2	$ 4.3	$ 14.5	$ 7.4
Florida | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	1.90%	2.70%	1.70%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 7.4	$ 6.8	$ 13.9	$ 12.4
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	3.00%	2.60%	2.90%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 71.8	$ 52.7	$ 134.9	$ 101.8
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.20%	23.30%	25.20%	23.50%
US
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 274.9	$ 222.8	$ 519.7	$ 425.3
US | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	96.60%	98.50%	96.50%	98.50%
Europe and UK
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.6	$ 3.4	$ 19.0	$ 6.5
Europe and UK | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.40%	1.50%	3.50%	1.50%