Stock-based Compensation - Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 11.9	$ 2.4	$ 18.0	$ 4.6
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	10.5	2.4	16.0	4.6
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.4	0.0	2.0	0.0
Loss and loss adjustment expense, net
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0.5	0.1	0.7	0.1
Other insurance expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0.2	0.2	0.4	0.4
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.3	0.6	2.4	1.3
Technology development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	6.8	0.7	7.5	1.3
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 3.1	$ 0.8	$ 7.0	$ 1.5