Geographical Breakdown of Gross Written Premium (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			1
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 116.8	$ 71.2	$ 282.1	$ 156.0
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	$ 29.0	$ 15.6	$ 70.4	$ 34.9
California | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	24.80%	21.90%	25.00%	22.40%
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 21.7	$ 15.9	$ 55.4	$ 36.5
Texas | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	18.60%	22.30%	19.60%	23.40%
New York
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 14.3	$ 8.6	$ 35.2	$ 18.8
New York | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	12.20%	12.10%	12.50%	12.10%
Georgia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 4.9	$ 3.8	$ 12.9	$ 8.7
Georgia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.20%	5.30%	4.60%	5.60%
Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.0	$ 3.4	$ 12.1	$ 7.6
Illinois | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.30%	4.80%	4.30%	4.90%
New Jersey
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 5.3	$ 3.0	$ 12.1	$ 6.1
New Jersey | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	4.50%	4.20%	4.30%	3.90%
Pennsylvania
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.3	$ 2.0	$ 7.4	$ 3.8
Pennsylvania | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.80%	2.80%	2.60%	2.40%
Colorado
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 3.1	$ 1.5	$ 6.5	$ 3.0
Colorado | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.70%	2.10%	2.30%	1.90%
Virginia
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.6	$ 1.4	$ 6.0	$ 2.7
Virginia | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.20%	2.00%	2.10%	1.70%
Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 2.4	$ 2.0	$ 5.9	$ 4.4
Michigan | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	2.10%	2.80%	2.10%	2.80%
All other
Revenues from External Customers and Long-Lived Assets [Line Items]
Amount	$ 25.2	$ 14.0	$ 58.2	$ 29.5
All other | Gross written premium | Geographic Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
% of GWP	21.60%	19.70%	20.60%	18.90%