Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions			9 Months Ended	12 Months Ended
	Jul. 02, 2020	Feb. 18, 2020	Sep. 30, 2020	Dec. 31, 2019	Jul. 07, 2020
Sale of Stock [Line Items]
Offering costs, expensed				$ 3.5
Common stock, authorized (shares)			200,000,000	52,000,000	200,000,000
Common stock, par value (usd per share)			$ 0.00001	$ 0.00001	$ 0.00001
Preferred stock, authorized (shares)			10,000,000		10,000,000
Preferred stock, par value (usd per share)					$ 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