Segment Information - Gross Written Premium by State (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 929.0	$ 738.4	$ 555.7
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 225.6	$ 191.6	$ 142.0
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.30%	25.90%	25.60%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 133.7	$ 117.5	$ 91.3
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	14.40%	15.90%	16.40%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 96.3	$ 80.8	$ 66.0
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	10.40%	10.90%	11.90%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 44.3	$ 37.6	$ 28.3
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.80%	5.10%	5.10%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 43.7	$ 35.3	$ 26.3
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.70%	4.80%	4.70%
Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 38.9	$ 25.9	$ 15.8
Washington | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.20%	3.50%	2.80%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 29.2	$ 22.0	$ 15.8
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.10%	3.00%	2.80%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 27.2	$ 22.6	$ 19.8
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.90%	3.10%	3.60%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 24.9	$ 19.7	$ 14.4
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.70%	2.70%	2.60%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 22.4	$ 18.0	$ 12.4
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.40%	2.20%
All others
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 222.8	$ 158.1	$ 119.2
All others | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	23.90%	21.40%	21.50%
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 909.0	$ 729.1	$ 551.3
United States | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	97.80%	98.70%	99.20%
Europe and U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 20.0	$ 9.3	$ 4.4
Europe and U.K. | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.20%	1.30%	0.80%