Fair Value of Financial Instruments - Schedule of Calculation of Estimated Fair Value of Success Payment Liabilities (Details) - Success Payment Liabilities	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Fair Value of Common Stock \| Harvard
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability measurement input	24.27	24.8
Fair Value of Common Stock \| Broad Institute
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability measurement input	24.27	24.8
Expected Volatility \| Harvard
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability measurement input	0.74	0.78
Expected Volatility \| Broad Institute
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability measurement input	0.75	0.81
Expected Term (years) \| Minimum \| Harvard
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, term	7 days	10 days
Expected Term (years) \| Minimum \| Broad Institute
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, term	7 days	10 days
Expected Term (years) \| Maximum \| Harvard
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, term	3 years 8 months 26 days	4 years 5 months 26 days
Expected Term (years) \| Maximum \| Broad Institute
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Success payment liability, term	4 years 7 months 9 days	5 years 4 months 9 days

Beam Therapeutics Inc. 10-Q