Geographical Breakdown of Gross Written Premium (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Gross Written Premium [Line Items]
Amount	$ 229.5	$ 173.3	$ 663.6	$ 452.2
Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	100.00%	100.00%	100.00%	100.00%
Texas
Gross Written Premium [Line Items]
Amount	$ 42.5	$ 38.2	$ 126.3	$ 115.2
Texas \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	18.50%	22.00%	19.00%	25.50%
California
Gross Written Premium [Line Items]
Amount	$ 40.6	$ 32.8	$ 116.6	$ 86.1
California \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	17.70%	18.90%	17.70%	19.00%
Florida
Gross Written Premium [Line Items]
Amount	$ 29.9	$ 15.1	$ 89.4	$ 37.8
Florida \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	13.00%	8.70%	13.50%	8.40%
Georgia
Gross Written Premium [Line Items]
Amount	$ 8.5	$ 7.3	$ 30.7	$ 20.9
Georgia \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	3.70%	4.20%	4.60%	4.60%
Colorado
Gross Written Premium [Line Items]
Amount	$ 5.6	$ 5.5	$ 18.1	$ 12.7
Colorado \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.40%	3.20%	2.70%	2.80%
Arizona
Gross Written Premium [Line Items]
Amount	$ 5.0	$ 3.9	$ 14.1	$ 10.3
Arizona \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.20%	2.40%	2.10%	2.30%
Illinois
Gross Written Premium [Line Items]
Amount	$ 7.5	$ 6.6	$ 19.9	$ 16.0
Illinois \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	3.30%	3.80%	3.00%	3.50%
New Jersey
Gross Written Premium [Line Items]
Amount	$ 4.5	$ 4.7	$ 12.8	$ 9.2
New Jersey \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.00%	2.70%	1.90%	2.00%
Ohio
Gross Written Premium [Line Items]
Amount	$ 3.3	$ 3.9	$ 11.2	$ 10.3
Ohio \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	1.40%	2.40%	1.70%	2.30%
Missouri
Gross Written Premium [Line Items]
Amount	$ 5.1	$ 4.7	$ 12.6	$ 11.3
Missouri \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.20%	2.70%	1.90%	2.50%
Other
Gross Written Premium [Line Items]
Amount	$ 77.0	$ 50.6	$ 211.9	$ 122.4
Other \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	33.60%	29.20%	31.90%	27.10%