Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 75.8	$ 38.1
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 19.2	$ 9.4
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.30%	24.70%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 15.0	$ 8.8
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	19.80%	23.10%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.0	$ 4.9
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	13.20%	12.90%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.7	$ 2.1
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.90%	5.50%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.1	$ 1.5
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.10%	3.90%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.0	$ 1.6
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.00%	4.20%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.8	$ 0.7
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	1.80%
Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.6	$ 1.0
Michigan | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.60%
Ohio
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.6	$ 1.0
Ohio | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.60%
Maryland
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.6	$ 0.8
Maryland | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.10%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 15.2	$ 6.3
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	20.00%	16.60%