CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Millions	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance (shares) at Dec. 31, 2018	24,445,555
Beginning balance at Dec. 31, 2018	$ 180.8
Convertible Preferred Stock
Issuance of preferred stock (shares)	3,622
Ending balance (shares) at Mar. 31, 2019	24,449,177
Ending balance at Mar. 31, 2019	$ 180.8
Beginning balance (shares) at Dec. 31, 2018		10,983,684
Beginning balance at Dec. 31, 2018	(79.1)	$ 0.0	$ 10.7	$ (89.8)	$ 0.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)		3,125
Stock-based compensation	0.4		0.4
Net loss	(21.6)			(21.6)
Other comprehensive income	0.0				0.0
Ending balance (shares) at Mar. 31, 2019		10,986,809
Ending balance at Mar. 31, 2019	$ (100.3)	$ 0.0	11.1	(111.4)	0.0
Beginning balance (shares) at Dec. 31, 2018	24,445,555
Beginning balance at Dec. 31, 2018	$ 180.8
Ending balance (shares) at Sep. 30, 2019	31,557,107
Ending balance at Sep. 30, 2019	$ 480.2
Beginning balance (shares) at Dec. 31, 2018		10,983,684
Beginning balance at Dec. 31, 2018	(79.1)	$ 0.0	10.7	(89.8)	0.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Net loss	(75.8)
Ending balance (shares) at Sep. 30, 2019		11,263,261
Ending balance at Sep. 30, 2019	$ (152.2)	$ 0.0	13.5	(165.6)	(0.1)
Beginning balance (shares) at Mar. 31, 2019	24,449,177
Beginning balance at Mar. 31, 2019	$ 180.8
Convertible Preferred Stock
Issuance of preferred stock (shares)	4,146,294
Issuance of preferred stock, net of issuance costs	$ 174.4
Ending balance (shares) at Jun. 30, 2019	28,595,471,000,000
Ending balance at Jun. 30, 2019	$ 355.2
Beginning balance (shares) at Mar. 31, 2019		10,986,809
Beginning balance at Mar. 31, 2019	(100.3)	$ 0.0	11.1	(111.4)	0.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)		97,625
Exercise of stock options	0.3		0.3
Stock-based compensation	0.4		0.4
Net loss	(23.1)			(23.1)
Other comprehensive income	0.0				0.0
Ending balance (shares) at Jun. 30, 2019		11,084,434
Ending balance at Jun. 30, 2019	$ (122.7)	$ 0.0	11.8	(134.5)	0.0
Convertible Preferred Stock
Issuance of preferred stock (shares)	2,961,636
Issuance of preferred stock, net of issuance costs	$ 125.0
Ending balance (shares) at Sep. 30, 2019	31,557,107
Ending balance at Sep. 30, 2019	$ 480.2
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)		73,340
Exercise of stock options	0.1		0.1
Release of shares upon repayment of partial recourse loan (shares)		105,487
Release of shares upon repayment of partial recourse loan	0.1		0.1
Stock-based compensation	1.5		1.5
Net loss	(31.1)			(31.1)
Other comprehensive income	(0.1)				(0.1)
Ending balance (shares) at Sep. 30, 2019		11,263,261
Ending balance at Sep. 30, 2019	$ (152.2)	$ 0.0	13.5	(165.6)	(0.1)
Beginning balance (shares) at Dec. 31, 2019	31,557,107
Beginning balance at Dec. 31, 2019	$ 480.2
Ending balance (shares) at Mar. 31, 2020	31,557,107
Ending balance at Mar. 31, 2020	$ 480.2
Beginning balance (shares) at Dec. 31, 2019	11,271,228	11,271,228
Beginning balance at Dec. 31, 2019	$ (182.5)	$ 0.0	15.7	(198.3)	0.1
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)		54,374
Stock-based compensation	2.2		2.2
Contribution to the Lemonade Foundation (shares)		500,000
Contribution to the Lemonade Foundation	12.2		12.2
Net loss	(36.5)			(36.5)
Other comprehensive income	0.0				0.0
Ending balance (shares) at Mar. 31, 2020		11,825,602
Ending balance at Mar. 31, 2020	$ (204.6)	$ 0.0	30.1	(234.8)	0.1
Beginning balance (shares) at Dec. 31, 2019	31,557,107
Beginning balance at Dec. 31, 2019	$ 480.2
Convertible Preferred Stock
Conversion of Convertible Preferred Stock to common stock upon closing of initial public offering (shares)	(31,557,107)
Ending balance (shares) at Sep. 30, 2020	0
Ending balance at Sep. 30, 2020	$ 0.0
Beginning balance (shares) at Dec. 31, 2019	11,271,228	11,271,228
Beginning balance at Dec. 31, 2019	$ (182.5)	$ 0.0	15.7	(198.3)	0.1
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)	92,157
Net loss	$ (88.4)
Ending balance (shares) at Sep. 30, 2020	56,584,029	56,584,029
Ending balance at Sep. 30, 2020	$ 569.0	$ 0.0	855.0	(286.7)	0.7
Beginning balance (shares) at Mar. 31, 2020	31,557,107
Beginning balance at Mar. 31, 2020	$ 480.2
Ending balance (shares) at Jun. 30, 2020	31,557,107
Ending balance at Jun. 30, 2020	$ 480.2
Beginning balance (shares) at Mar. 31, 2020		11,825,602
Beginning balance at Mar. 31, 2020	(204.6)	$ 0.0	30.1	(234.8)	0.1
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)		30,562
Exercise of stock options	0.1		0.1
Stock-based compensation	2.4		2.4
Release of shares upon repayment (shares)		513,537
Release of shares upon repayment	1.3		1.3
Net loss	(21.0)			(21.0)
Other comprehensive income	0.2				0.2
Ending balance (shares) at Jun. 30, 2020		12,369,701
Ending balance at Jun. 30, 2020	$ (221.6)	$ 0.0	33.9	(255.8)	0.3
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 Sep. 30, 2020	0
Ending balance at Sep. 30, 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 $28.9 million	338.0		338.0
Exercise of stock options (shares)		7,221
Exercise of stock options	0.2		0.2
Stock-based compensation	2.7		2.7
Net loss	(30.9)			(30.9)
Other comprehensive income	$ 0.4				0.4
Ending balance (shares) at Sep. 30, 2020	56,584,029	56,584,029
Ending balance at Sep. 30, 2020	$ 569.0	$ 0.0	$ 855.0	$ (286.7)	$ 0.7