Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 24.5	$ 15.4	$ 45.7	$ 25.7
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	5.2	5.8	11.6	12.3
RSUs with service condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	17.4	9.6	31.6	18.6
RSUs with market condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.9	0.0	2.5	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	12.3	6.5	22.2	13.1
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	1.2	1.1
Location, Statement of Income, Balance [Axis]: us-gaap:ResearchAndDevelopmentExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	9.8	6.9	18.7	13.7
Location, Statement of Income, Balance [Axis]: us-gaap:SellingAndMarketingExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1.0	0.8	2.1	(3.5)
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	$ 0.6	$ 1.5	$ 1.3