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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions |  | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Feb. 18, 2018 | Sep. 30, 2020 | Dec. 31, 2019 | Jul. 07, 2020 | Dec. 31, 2018
Sale of Stock [Line Items] |  |  |  |  |
Offering costs, expensed |  |  | $ 3.5 |  |
Common stock, authorized (shares) |  | 200,000,000 | 52,000,000 | 200,000,000 | 42,000,000
Common stock, par value (usd per share) |  | $ 0.00001 | $ 0.00001 | $ 0.00001 | $ 0.00001
Preferred stock, authorized (shares) |  | 10,000,000 |  | 10,000,000 |
Preferred Stock, Shares Issued |  | 0 |  |  |
Preferred Stock, Shares Outstanding |  | 0 |  |  |
Preferred stock, par value (usd per share) |  | $ 0.00001 |  | $ 0.00001 |
IPO |  |  |  |  |
Sale of Stock [Line Items] |  |  |  |  |
Common stock sold (shares) |  |  | 12,650,000 |  |
Common stock sold, price (usd per share) |  |  | $ 29.00 |  |
Net proceeds from sale |  |  | $ 335.6 |  |
Offering costs, expensed |  |  | $ 3.5 |  |
Underwriter's option |  |  |  |  |
Sale of Stock [Line Items] |  |  |  |  |
Common stock sold (shares) |  |  | 1,650,000 |  |
Affiliated entity | The Lemonade Foundation |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Contribution to the Lemonade Foundation (shares) | 500,000 | 500,000 |  |  |

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