Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 126.1	$ 89.5	$ 236.7	$ 165.3
Number of reportable segments | segment			1
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	$ 28.3	$ 22.2	$ 55.4	$ 41.4
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.40%	24.80%	23.40%	25.00%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 24.1	$ 18.7	$ 44.3	$ 33.7
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	19.10%	20.90%	18.70%	20.40%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 15.9	$ 10.9	$ 30.7	$ 20.9
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.60%	12.20%	13.00%	12.60%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.9	$ 3.7	$ 11.2	$ 6.8
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.70%	4.10%	4.70%	4.10%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.6	$ 4.1	$ 11.0	$ 7.1
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.20%	4.60%	4.60%	4.30%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.0	$ 4.3	$ 9.4	$ 8.0
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.00%	4.80%	4.00%	4.80%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.9	$ 2.0	$ 6.9	$ 3.4
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.10%	2.20%	2.90%	2.10%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.2	$ 2.3	$ 5.9	$ 4.1
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.50%	2.60%	2.50%	2.50%
Virginia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.7	$ 1.9	$ 5.1	$ 3.4
Virginia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.10%	2.20%	2.10%
Maryland
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.5	$ 1.8	$ 4.7	$ 3.4
Maryland | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.00%	2.00%	2.00%	2.10%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 28.0	$ 17.6	$ 52.1	$ 33.1
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.30%	19.70%	22.00%	20.00%