Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements and the related disclosures are unaudited and have been prepared in conformity with the accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of Neon Therapeutics, Inc. and its wholly owned subsidiary, Neon Securities Corporation. All intercompany transactions and balances have been eliminated. The Company consolidates entities in which it has a controlling financial interest. Additionally, certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Accordingly, these interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2019, and notes thereto, which are contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the Securities and Exchange Commission (the "SEC") on March 2, 2020 (the “Annual Report on Form 10-K”). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements and, in the opinion of management, reflect all normal recurring adjustments considered necessary for a fair presentation of the Company’s financial position as of March 31, 2020, and the results of its operations for the three months ended March 31, 2020 and 2019, and its cash flows for the three months ended March 31, 2020 and 2019. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from the Company's audited financial statements, but does not contain all of the footnote disclosures required by GAAP. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results that may be expected for the full year or any other subsequent interim period. Summary of Significant Accounting Policies The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2020. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates of accounting reflected in these condensed consolidated financial statements include, but are not limited to, estimates related to accrued expenses, the valuation of common stock prior to the completion of the Company's IPO, stock-based compensation, the present value of lease liabilities and the corresponding right-of-use assets and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions. Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board ("FASB") issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"), which eliminates, adds and modifies certain disclosure requirements for fair value measurements. The Company adopted ASU 2018-13 as of the effective date January 1, 2020. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements or disclosures. In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract ("ASU 2018-15"), which provides additional guidance on the accounting for costs of implementation activities performed in a cloud computing arrangement that is a service contract. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The internal-use software guidance requires that certain costs incurred during the application development stage be capitalized and other costs incurred during the preliminary project stage and post-implementation stage be expensed as they are incurred. The accounting for the service element of a hosting arrangement that is a service contract is not affected. The Company adopted ASU 2018-15 as of the effective date of January 1, 2020 on a prospective basis. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). This standard requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The standard also establishes additional disclosure requirements related to credit risks. This guidance was originally effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption was permitted. In November 2019, the FASB subsequently issued ASU 2019-10, Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates, whereby the effective date of this standard for smaller reporting companies was deferred to annual reporting periods beginning after December 15, 2022, including interim periods within those annual reporting periods, and early adoption is still permitted. Accordingly, the Company will now adopt this standard effective January 1, 2023, and it is currently evaluating the potential impact that ASU 2016-13 may have on the condensed consolidated financial statements. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify various aspects related to the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020, with early adoption permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this standard.

Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation and Consolidation

The accompanying condensed consolidated financial statements and the related disclosures are unaudited and have been prepared in conformity with the accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of Neon Therapeutics, Inc. and its wholly owned subsidiary, Neon Securities Corporation. All intercompany transactions and balances have been eliminated. The Company consolidates entities in which it has a controlling financial interest.

Additionally, certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. Accordingly, these interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2019, and notes thereto, which are contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the Securities and Exchange Commission (the "SEC") on March 2, 2020 (the “Annual Report on Form 10-K”).

The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements and, in the opinion of management, reflect all normal recurring adjustments considered necessary for a fair presentation of the Company’s financial position as of March 31, 2020, and the results of its operations for the three months ended March 31, 2020 and 2019, and its cash flows for the three months ended March 31, 2020 and 2019. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from the Company's audited financial statements, but does not contain all of the footnote disclosures required by GAAP. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results that may be expected for the full year or any other subsequent interim period.

Summary of Significant Accounting Policies

The significant accounting policies and estimates used in the preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2019, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes in the Company’s significant accounting policies during the three months ended March 31, 2020.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates of accounting reflected in these condensed consolidated financial statements include, but are not limited to, estimates related to accrued expenses, the valuation of common stock prior to the completion of the Company's IPO, stock-based compensation, the present value of lease liabilities and the corresponding right-of-use assets and income taxes. The Company bases its estimates on historical experience and other market specific or other relevant assumptions it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates, as there are changes in circumstances, facts and experience. Actual results could differ from those estimates or assumptions.

Recently Adopted Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board ("FASB") issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement ("ASU 2018-13"), which eliminates, adds and modifies certain disclosure requirements for fair value measurements. The Company adopted ASU 2018-13 as of the effective date January 1, 2020. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements or disclosures.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract ("ASU 2018-15"), which provides additional guidance on the accounting for costs of implementation activities performed in a cloud computing arrangement that is a service contract. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The internal-use software guidance requires that certain costs incurred during the application development stage be capitalized and other costs incurred during the preliminary project stage and post-implementation stage be expensed as they are incurred. The accounting for the service element of a hosting arrangement that is a service contract is not affected. The Company adopted ASU 2018-15 as of the effective date of January 1, 2020 on a prospective basis. The adoption of the standard did not have a material impact on the Company's condensed consolidated financial statements.

Recently Issued Accounting Pronouncements

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). This standard requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. It also limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The standard also establishes additional disclosure requirements related to credit risks. This guidance was originally effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods, and early adoption was permitted. In November 2019, the FASB subsequently issued ASU 2019-10, Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates, whereby the effective date of this standard for smaller reporting companies was deferred to annual reporting periods beginning after December 15, 2022, including interim periods within those annual reporting periods, and early adoption is still permitted. Accordingly, the Company will now adopt this standard effective January 1, 2023, and it is currently evaluating the potential impact that ASU 2016-13 may have on the condensed consolidated financial statements.

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("ASU 2019-12"), which is intended to simplify various aspects related to the accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in Topic 740 and also clarifies and amends existing guidance to improve consistent application. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020, with early adoption permitted. The Company does not anticipate a material impact to the condensed consolidated financial statements as a result of the adoption of this standard.