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Related Party Transactions (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 18, 2018 shares | Sep. 30, 2020 USD ($) director | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) director $ / shares shares | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Due to (from) related parties |  | $ 0.0 |  | $ 0.0 |  | $ 0.0 | $ 0.0
Common share contribution to the Lemonade Foundation |  |  |  | 12.2 |  |  |
Key stockholder | Travel related expenses |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Expenses with related parties |  | $ 0.0 | $ 0.1 | $ 0.1 | $ 0.2 | 0.3 | 0.2
Affiliated entity | The Lemonade Foundation |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Number of shared directors | director |  | 2 |  | 2 |  |  |
Contribution to the Lemonade Foundation (shares) | shares | 500,000 |  |  | 500,000 |  |  |
Contribution of common stock to related party, fair value (usd per share) | $ / shares |  |  |  | $ 24.36 |  |  |
Common share contribution to the Lemonade Foundation |  |  |  | $ 12.2 |  |  |
Due from related party |  | $ 0.1 |  | 0.1 |  |  |
Affiliated entity | Rental expense |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |
Expenses with related parties |  | $ 0.1 |  | $ 0.1 |  | $ 0.1 | $ 0.1

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