CONSOLIDATED STATEMENTS OF CHANGES IN CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Millions	Total	Series C Preferred Stock	Series D Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance (shares) at Dec. 31, 2017	15,671,730
Beginning balance at Dec. 31, 2017	$ 60.4
Convertible Preferred Stock
Issuance of preferred stock (shares)		8,700,224
Issuance of preferred stock, net of issuance costs		$ 119.8
Exercise of Series A Preferred stock warrants (shares)	73,601
Exercise of Series A Preferred stock warrants	$ 0.6
Ending balance (shares) at Dec. 31, 2018	24,445,555
Ending balance at Dec. 31, 2018	$ 180.8
Beginning balance (shares) at Dec. 31, 2017				10,887,059
Beginning balance at Dec. 31, 2017	(28.4)			$ 0.0	$ 8.5	$ (36.9)	$ 0.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Repayment of partial recourse loan and release of shares upon repayment (shares)				62,500
Repayment of partial recourse loan and release of shares upon repayment	0.1				0.1
Exercise of stock options (shares)				34,125
Stock-based compensation	2.1				2.1
Net loss	(52.9)					(52.9)
Ending balance (shares) at Dec. 31, 2018				10,983,684
Ending balance at Dec. 31, 2018	$ (79.1)			$ 0.0	10.7	(89.8)	0.0
Convertible Preferred Stock
Issuance of preferred stock (shares)		3,622	7,107,930
Issuance of preferred stock, net of issuance costs		$ 0.0	$ 299.4
Ending balance (shares) at Dec. 31, 2019	31,557,107	8,703,846	7,107,930
Ending balance at Dec. 31, 2019	$ 480.2	$ 119.8	$ 299.4
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Repayment of partial recourse loan and release of shares upon repayment (shares)				105,487
Repayment of partial recourse loan and release of shares upon repayment	0.2				0.2
Exercise of stock options (shares)				182,057
Exercise of stock options	0.5				0.5
Stock-based compensation	4.3				4.3
Net loss	(108.5)					(108.5)
Other comprehensive income	$ 0.1						0.1
Ending balance (shares) at Dec. 31, 2019	11,271,228			11,271,228
Ending balance at Dec. 31, 2019	$ (182.5)			$ 0.0	15.7	(198.3)	0.1
Convertible Preferred Stock
Conversion of Convertible Preferred Stock to common stock upon closing of initial public offering (shares)	(31,557,107)
Conversion of Convertible Preferred Stock to common stock upon closing of initial public offering	$ (480.2)
Ending balance (shares) at Dec. 31, 2020	0
Ending balance at Dec. 31, 2020	$ 0.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Conversion of convertible preferred stock to common stock upon closing of initial public offering (shares)				31,557,107
Conversion of convertible preferred stock to common stock upon closing of initial public offering	480.2				480.2
Issuance of common stock upon closing of initial public offering (shares)				12,650,000
Issuance of common stock upon closing of initial public offering, net of issuance costs and underwriting fees of $31.3 million	338.0				338.0
Contribution to the Lemonade Foundation (shares)				500,000
Contribution to the Lemonade Foundation	12.2				12.2
Repayment of partial recourse loan and release of shares upon repayment (shares)				513,537
Repayment of partial recourse loan and release of shares upon repayment	$ 1.3				1.3
Exercise of stock options (shares)	282,422			282,422
Exercise of stock options	$ 1.8				1.8
Stock-based compensation	10.6				10.6
Net loss	(122.3)					(122.3)
Other comprehensive income	$ 1.7						1.7
Ending balance (shares) at Dec. 31, 2020	56,774,294			56,774,294
Ending balance at Dec. 31, 2020	$ 541.0			$ 0.0	$ 859.8	$ (320.6)	$ 1.8