Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 23.2	$ 17.1	$ 68.9	$ 42.8
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	4.1	6.1	15.7	18.4
RSUs with service condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	17.2	11.0	48.8	29.6
RSUs with market condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.9	0.0	4.4	0.0
Warrant shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0.0	0.0	0.0	(5.2)
Location, Statement of Income, Balance [Axis]: us-gaap:GeneralAndAdministrativeExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	11.1	7.2	33.3	20.3
Location, Statement of Income, Balance [Axis]: us-gaap:LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0.6	0.6	1.8	1.7
Location, Statement of Income, Balance [Axis]: us-gaap:ResearchAndDevelopmentExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	9.6	7.4	28.3	21.1
Location, Statement of Income, Balance [Axis]: us-gaap:SellingAndMarketingExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.1	1.1	3.2	(2.4)
Location, Statement of Income, Balance [Axis]: lmnd:OtherInsuranceExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0.8	$ 0.8	$ 2.3	$ 2.1