Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 89.5	$ 46.7	$ 165.3	$ 84.8
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	$ 22.2	$ 9.9	$ 41.4	$ 19.3
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.80%	21.20%	25.00%	22.80%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 18.7	$ 11.8	$ 33.7	$ 20.6
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	20.90%	25.30%	20.40%	24.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 10.9	$ 5.3	$ 20.9	$ 10.2
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.20%	11.30%	12.60%	12.00%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.3	$ 2.8	$ 8.0	$ 4.9
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.80%	6.00%	4.80%	5.80%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.1	$ 2.6	$ 7.1	$ 4.2
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.60%	5.60%	4.30%	5.00%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.7	$ 1.6	$ 6.8	$ 3.1
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.10%	3.40%	4.10%	3.70%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.3	$ 1.1	$ 4.1	$ 1.8
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	2.40%	2.50%	2.10%
Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.9	$ 1.4	$ 3.5	$ 2.4
Michigan | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	3.00%	2.10%	2.80%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.0	$ 0.8	$ 3.4	$ 1.5
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.20%	1.70%	2.10%	1.80%
Maryland
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.8	$ 0.9	$ 3.4	$ 1.7
Maryland | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.00%	1.90%	2.10%	2.00%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 17.6	$ 8.5	$ 33.0	$ 15.1
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	19.70%	18.20%	20.00%	17.70%