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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

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