Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 46.7	$ 25.4	$ 84.8	$ 44.7
Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	100.00%	100.00%	100.00%	100.00%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 11.8	$ 6.7	$ 20.6	$ 12.0
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	25.30%	26.40%	24.30%	26.80%
California
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 9.9	$ 6.4	$ 19.3	$ 11.3
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.20%	25.20%	22.80%	25.30%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.3	$ 3.6	$ 10.2	$ 6.5
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	11.30%	14.20%	12.00%	14.50%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.8	$ 1.4	$ 4.9	$ 2.4
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	6.00%	5.50%	5.80%	5.40%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.6	$ 1.3	$ 4.2	$ 2.1
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	5.60%	5.10%	5.00%	4.70%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.6	$ 1.0	$ 3.1	$ 1.9
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.40%	3.90%	3.70%	4.30%
Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.4	$ 0.5	$ 2.4	$ 1.0
Michigan | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	3.00%	1.90%	2.80%	2.20%
Ohio
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.2	$ 0.6	$ 2.2	$ 1.2
Ohio | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.60%	2.40%	2.60%	2.70%
Arizona
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.1	$ 0.5	$ 1.9	$ 0.9
Arizona | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.00%	2.20%	2.00%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 1.1	$ 0.6	$ 1.8	$ 1.0
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.40%	2.40%	2.10%	2.20%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 7.9	$ 2.8	$ 14.2	$ 4.4
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	16.80%	11.00%	16.70%	9.90%