Related Party Transactions (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2025 USD ($) director $ / shares shares	Dec. 31, 2020 shares	Dec. 31, 2024 USD ($) shares
Related Party Transaction [Line Items]
Due from related party (less than for December 31, 2024)	$ 35,700,000		$ 42,400,000
Due to related party (less than for December 31,2024)	$ 107,000,000.0		105,700,000
The Lemonade Foundation | Affiliated entity
Related Party Transaction [Line Items]
Contribution to the lemonade foundation (in shares) | shares	500,000	500,000
Contribution of common stock to related party, fair value (usd per share) | $ / shares	$ 24.36
Due from related party (less than for December 31, 2024)	$ 0		100,000
Due to related party (less than for December 31,2024)	$ 0		$ 100,000
The Lemonade Foundation | Affiliated entity | Lemonade, Inc.
Related Party Transaction [Line Items]
Shares owned by foundation (in shares) | shares	400,000		400,000
Related Party
Related Party Transaction [Line Items]
Number of shared directors | director	2