Geographical Breakdown of Gross Written Premium (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Gross Written Premium [Line Items]
Amount	$ 257.6	$ 244.8	$ 452.4	$ 434.2
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%
California
Gross Written Premium [Line Items]
Amount	$ 60.0	$ 43.2	$ 100.5	$ 76.0
California \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	23.30%	17.70%	22.20%	17.50%
Texas
Gross Written Premium [Line Items]
Amount	$ 36.7	$ 44.2	$ 64.4	$ 83.8
Texas \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	14.20%	18.20%	14.20%	19.30%
Florida
Gross Written Premium [Line Items]
Amount	$ 37.1	$ 38.0	$ 62.7	$ 59.5
Florida \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	14.40%	15.50%	13.90%	13.70%
Illinois
Gross Written Premium [Line Items]
Amount	$ 8.9	$ 7.3	$ 14.9	$ 12.4
Illinois \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	3.50%	3.00%	3.30%	2.90%
Georgia
Gross Written Premium [Line Items]
Amount	$ 6.3	$ 13.0	$ 13.6	$ 22.2
Georgia \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.40%	5.30%	3.00%	5.10%
South Carolina
Gross Written Premium [Line Items]
Amount	$ 8.9	$ 5.7	$ 13.3	$ 9.2
South Carolina \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	3.50%	2.30%	2.90%	2.10%
New York
Gross Written Premium [Line Items]
Amount	$ 6.9	$ 5.7	$ 12.6	$ 9.4
New York \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.70%	2.30%	2.80%	2.20%
Colorado
Gross Written Premium [Line Items]
Amount	$ 5.1	$ 6.7	$ 9.2	$ 12.5
Colorado \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	2.00%	2.70%	2.00%	2.90%
New Jersey
Gross Written Premium [Line Items]
Amount	$ 4.5	$ 4.0	$ 9.1	$ 8.3
New Jersey \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	1.70%	1.60%	2.00%	1.90%
Ohio
Gross Written Premium [Line Items]
Amount	$ 4.4	$ 4.2	$ 9.0	$ 7.9
Ohio \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	1.70%	1.70%	2.00%	1.80%
Other
Gross Written Premium [Line Items]
Amount	$ 78.8	$ 72.8	$ 143.1	$ 133.0
Other \| Geographic Concentration Risk \| Gross written premium
Gross Written Premium [Line Items]
Concentration risk, percentage (as a percent)	30.60%	29.70%	31.70%	30.60%