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, 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
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)
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
Ending balance (shares) at Jun. 30, 2020	31,557,107
Ending balance at Jun. 30, 2020	$ 480.2
Beginning balance (shares) at Dec. 31, 2019		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]
Net loss	(57.5)
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
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	0.1		0.1
Exercise of stock options (shares)		30,562
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
Beginning balance (shares) at Dec. 31, 2020	0
Beginning balance at Dec. 31, 2020	$ 0.0
Ending balance (shares) at Mar. 31, 2021	0
Ending balance at Mar. 31, 2021	$ 0.0
Beginning balance (shares) at Dec. 31, 2020	56,774,294	56,774,294
Beginning balance at Dec. 31, 2020	$ 541.0	$ 0.0	859.8	(320.6)	1.8
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Issuance of common stock upon closing of Follow-On Offering (shares)		4,018,647
Issuance of common stock upon closing of Follow-on Offering, net of underwriting discounts and commissions and offering costs of $22.8 million	640.3		640.3
Exercise of stock options	6.1		6.1
Exercise of stock options (shares)		577,162
Stock-based compensation	6.1		6.1
Net loss	(49.0)			(49.0)
Other comprehensive income	(0.8)				(0.8)
Ending balance (shares) at Mar. 31, 2021		61,370,103
Ending balance at Mar. 31, 2021	$ 1,143.7	$ 0.0	1,512.3	(369.6)	1.0
Beginning balance (shares) at Dec. 31, 2020	0
Beginning balance at Dec. 31, 2020	$ 0.0
Ending balance (shares) at Jun. 30, 2021	0
Ending balance at Jun. 30, 2021	$ 0.0
Beginning balance (shares) at Dec. 31, 2020	56,774,294	56,774,294
Beginning balance at Dec. 31, 2020	$ 541.0	$ 0.0	859.8	(320.6)	1.8
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options (shares)	739,186
Net loss	$ (104.6)
Ending balance (shares) at Jun. 30, 2021	61,532,127	61,532,127
Ending balance at Jun. 30, 2021	$ 1,101.6	$ 0.0	1,525.9	(425.2)	0.9
Beginning balance (shares) at Mar. 31, 2021	0
Beginning balance at Mar. 31, 2021	$ 0.0
Ending balance (shares) at Jun. 30, 2021	0
Ending balance at Jun. 30, 2021	$ 0.0
Beginning balance (shares) at Mar. 31, 2021		61,370,103
Beginning balance at Mar. 31, 2021	1,143.7	$ 0.0	1,512.3	(369.6)	1.0
Increase (Decrease) in Stockholders' Deficit [Roll Forward]
Exercise of stock options	1.7		1.7
Exercise of stock options (shares)		162,024
Stock-based compensation	11.9		11.9
Net loss	(55.6)			(55.6)
Other comprehensive income	$ (0.1)				(0.1)
Ending balance (shares) at Jun. 30, 2021	61,532,127	61,532,127
Ending balance at Jun. 30, 2021	$ 1,101.6	$ 0.0	$ 1,525.9	$ (425.2)	$ 0.9