Geographical Breakdown of Gross Written Premium (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 214.4	$ 115.8	$ 46.8
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 49.8	$ 29.0	$ 12.5
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	23.20%	25.00%	26.70%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 47.8	$ 28.6	$ 13.4
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	22.30%	24.70%	28.60%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 26.7	$ 15.8	$ 7.3
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.50%	13.60%	15.60%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 11.6	$ 6.2	$ 2.2
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.40%	5.40%	4.70%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.8	$ 5.2	$ 2.5
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.60%	4.50%	5.30%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 8.6	$ 4.7	$ 1.7
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.00%	4.10%	3.60%
Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 6.0	$ 3.2	$ 0.9
Michigan | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.80%	1.90%
Ohio
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.2	$ 2.9	$ 1.3
Ohio | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.50%	2.80%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.1	$ 2.7	$ 1.1
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.30%	2.40%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.6	$ 2.5	$ 0.8
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.20%	1.70%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 39.2	$ 15.0	$ 3.1
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	18.30%	12.90%	6.70%