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Stock Option and Grant Plan - Additional Information (Details) - USD ($) | 1 Months Ended | 6 Months Ended
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Feb. 29, 2020 | Jun. 30, 2021
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Weighted-average grant date fair value per share of options granted |  | $ 57.33
Unvested Stock Options |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Unrecognized compensation expense related to unvested stock options |  | $ 90,300,000
Unrecognized compensation cost, cost to recognize over a weighted-average period |  | 2 years 6 months
Restricted Stock |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Unrecognized compensation cost, cost to recognize over a weighted-average period |  | 2 years 2 months 12 days
Unrecognized stock-based compensation expense, expected to vest |  | $ 62,400,000
Restricted Stock | Minimum |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Vesting period |  | 2 years
Restricted Stock | Maximum |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Vesting period |  | 4 years
2019 Plan |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Number of shares reserved for future issuance |  | 9,188,648
Number of shares available for future issuance |  | 2,555,874
2019 Employee Stock Purchase Plan |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Number of shares available for future issuance |  | 1,049,460
Common stock, shares description |  | In February 2020, the Company’s board of directors adopted the Beam Therapeutics Inc. 2019 Employee Stock Purchase Plan, or ESPP, which was approved by the Company’s stockholders. Pursuant to the ESPP, certain employees of the Company, excluding consultants and non-employee directors, are eligible to purchase common stock of the Company at a reduced rate during offering periods. The ESPP permits participants to purchase common stock using funds contributed through payroll deductions, subject to a calendar year limit of $25,000 and at a purchase price of 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the applicable purchase date, which will be the final trading day of the applicable purchase period. The first offering period is expected to commence on October 1, 2021.
Maximum amount allowed to purchase common stock using payroll deduction | $ 25,000 |
Percentage of common stock purchase price lower of the fair market value | 85.00% |
Common stock, offering period, start date | Oct. 01, 2021 |

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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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