Reinsurance - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Apr. 30, 2021 reinsurer	May 01, 2020 agreement	Apr. 30, 2020 reinsurer	May 01, 2019 agreement	Apr. 30, 2019 reinsurer	May 01, 2018 agreement	Apr. 30, 2018 reinsurer	May 01, 2017 agreement
Effects of Reinsurance [Line Items]
Paid net	$ 9.0
Reinsurance ceding rate							85.00%		85.00%		85.00%		85.00%
Acquisition costs from reinsurance		$ 2.1	$ 0.0	$ 9.1	$ 4.7
Number Of Quota-Share Reinsurance Agreements | agreement							5		4		3		2
Number Of Unaffiliated Reinsurers | reinsurer						5		4		3		2
Metromile Insurance Company
Effects of Reinsurance [Line Items]
Paid net				$ 6.2