Stock-based Compensation - Additional Information (Details) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2025 shares	Jul. 02, 2020 shares	Jul. 31, 2015 employee shares	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($)	Dec. 31, 2022 $ / shares shares	Apr. 04, 2025 shares	Dec. 31, 2024	Apr. 30, 2024 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $				$ 17.1	$ 16.9	$ 42.8	$ 47.2
Remaining unvested warrant shares (in shares)				3,170,834		3,170,834
Omnibus Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period								5 years
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, stock options \| $				$ 42.1		$ 42.1
Unrecognized expense, period for recognition						1 year 1 month 6 days
Stock-based compensation expense \| $				6.1	7.7	$ 18.4	22.5
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, RSUs \| $				85.5		$ 85.5
Unrecognized expense, period for recognition						1 year 3 months 18 days
Grand date fair value (in dollars per share) \| $ / shares						$ 43.61
Stock-based compensation expense \| $				11.0	7.2	$ 29.6	20.2
Warrant shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $				$ 0.0	$ 2.0	$ (5.2)	$ 4.5
Warrant shares \| Omnibus Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period								5 years
Warrant, number of shares issuable, common stock (in shares)								3,352,025
Grand date fair value (in dollars per share) \| $ / shares								$ 20.37
Number of warrant shares vested (in shares)											181,191
Remaining unvested warrant shares (in shares)									3,170,834
2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)		5,503,678
Common stock reserved for issuance, annual increase, percentage of outstanding shares		5.00%								5.00%
Maximum shares that may be issued upon exercise of incentive stock options (in shares)		3,650,000
Additional shares authorized (in shares)	3,636,043
Common stock available for grant (in shares)				8,467,231		8,467,231
2020 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)		1,000,000
Common stock reserved for issuance, annual increase, percentage of outstanding shares		1.00%
Number of shares available for issuance, annual increase (in shares)		1,000,000
Annual increase in shares reserved for issuance (in shares)	0
Share based compensation arrangement by share based payment award, shares issued in period (in shares)						0
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)			7,312,590
Common stock available for grant (in shares)				0		0
Number of shares of common stock per exercised option (in shares) \| employee			1
2015 Plan \| Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Expiration period			10 years
Incentive Stock Plan 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation equity awards granted (in shares)								404,207

Lemonade, Inc. 10-Q