Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2022
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries Instil Bio (UK) Ltd. (formerly Immetacyte Ltd. (“Immetacyte”)) and Complex Therapeutics, LLC. Immetacyte was acquired on March 2, 2020 and Complex Therapeutics, LLC was formed on October 14, 2020. All intercompany balances and transactions have been eliminated in consolidation. The unaudited condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2022, the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2022 and 2021, the condensed consolidated statements of convertible preferred stock and stockholders' equity for the three months ended March 31, 2022 and 2021, and the results of its cash flows for the three months ended March 31, 2022 and 2021. The financial data and other information disclosed in these notes related to the three months ended March 31, 2022 and 2021 are also unaudited. The results for the three months ended March 31, 2022 are not necessarily indicative of results to be expected for the year ending December 31, 2022, any other periods, or any future year period. The Company has evaluated subsequent events through the date the condensed consolidated financial statements were issued. The accompanying unaudited condensed consolidated financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the Securities and Exchange Commission (“SEC”) on March 7, 2022.
Segments	Segments Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents, Restricted Cash and Marketable Securities The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents include amounts invested in money market accounts. Restricted cash consists of a money market account which serves as collateral for the Company’s employee corporate credit cards and is classified within other long-term assets on the consolidated balance sheets.
Marketable Securities	The Company's short-term marketable securities have original maturities of less than a year at date of purchase. The Company classifies and accounts for marketable securities as available-for-sale securities, which are carried at their fair values based on the quoted market prices of the securities. Unrealized gains and losses are reported as accumulated other comprehensive loss. Realized gains and losses on available-for-sale securities are included in net loss in the period earned or incurred. As of March 31, 2022 and December 31, 2021, marketable securities consisted of U.S. Treasury bills. Short-term marketable securities are recorded at their estimated fair value. The Company periodically reviews whether its securities may be other-than-temporarily impaired, including whether or not (i) the Company has the intent to sell the security or (ii) it is more likely than not that the Company will be required to sell the security before its anticipated recovery. If one of these factors is met, the Company will record an impairment loss associated with its impaired investment. The impairment loss will be recorded as a write-down of investments in the condensed consolidated balance sheets and a realized loss within other expense in the condensed consolidated statements of operations.
Leases	Leases The Company determines if an arrangement is or contains a lease at contract inception by assessing whether the arrangement contains an identified asset and whether the lessee has the right to control such asset. Lessees are required to classify leases as either finance or operating leases and to record a right-of-use (“ROU”) asset and a lease liability for all leases with a term greater than 12 months regardless of the lease classification. The lease classification will determine whether the lease expense is recognized based on an effective interest rate method or on a straight-line basis over the term of the lease. The Company determines the initial classification and measurement of its ROU assets and lease liabilities at the lease commencement date and thereafter if modified. For leases with a term greater than 12 months, the Company records the lease liability at the present value of lease payments over the term. The term of the Company’s leases equals the non-cancellable period of the lease, including any rent-free periods provided by the lessor, and also includes options to extend or terminate the lease that the Company is reasonably certain to exercise. The ROU asset equals the carrying amount of the related lease liability, adjusted for any lease payments made prior to lease commencement, any deferred rent upon adoption, and lease incentives provided by the lessor. The Company has elected, for all classes of underlying assets, not to recognize ROU assets and lease liabilities for leases with a term of 12 months or less. Lease cost for short-term leases is recognized on a straight-line basis over the lease term. The Company’s leases do not provide a readily determinable implicit rate. Therefore, the Company estimates its incremental borrowing rate to discount the lease payments based on information available at lease commencement. The Company determines its incremental borrowing rate based on the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. Variable lease payments are expensed as incurred and do not factor into the measurement of the applicable ROU asset or lease liability. Lease payments may be fixed or variable; however, only fixed payments are included in the Company’s lease liability calculation. Lease costs for the Company’s operating leases are recognized on a straight-line basis within operating expenses over the lease term. The Company’s lease agreements may contain non-lease components such as common area maintenance, operating expenses or other costs, which are expensed as incurred for all classes of assets.
Recent Accounting Pronouncements Adopted	Recent Accounting Pronouncements Adopted In February 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2016-02, Leases (Topic 842) which supersedes FASB ASC Topic 840, Leases (Topic 840) and provides principles for the recognition, measurement, presentation, and disclosure of leases for both lessees and lessors. The Company adopted the new standard effective January 1, 2022 using the modified retrospective transition approach. Upon adoption on January 1, 2022, the Company recognized ROU assets and lease liabilities totaling $15.4 million and $8.9 million, respectively, to reflect the present value of remaining lease payments under existing lease arrangements. The Company applied the modified retrospective transition approach and did not recast prior periods. As permitted by the standard, the Company elected the transition practical expedient package, which, among other things, allows the carryforward of historical lease classifications. The Company’s new accounting policies around leases are described in Leases, above, and in Note 6, Commitments and Contingencies. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. Additionally, the FASB issued ASU No. 2019-04, Codification Improvements to Topic 326, in April 2019 and ASU 2019-05, Financial Instruments — Credit Losses (Topic 326) — Targeted Transition Relief, in May 2019. The amendments affect loans, debt securities, trade receivables, net investments in leases, off-balance-sheet credit exposures, reinsurance receivables, and any other financial assets not excluded from the scope that have the contractual right to receive cash. The Company adopted this standard on January 1, 2022. The adoption of this standard did not have a material impact on the Company’s condensed consolidated financial statements and disclosures. In December 2019, the FASB issued ASU No. 2019-12. Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The standard simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and also improves consistent application by clarifying and amending existing guidance. The Company adopted this standard on January 1, 2022. The adoption of this standard did not have a material impact on the Company’s condensed consolidated financial statements and disclosures.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2022

The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries Instil Bio (UK) Ltd. (formerly Immetacyte Ltd. (“Immetacyte”)) and Complex Therapeutics, LLC. Immetacyte was acquired on March 2, 2020 and Complex Therapeutics, LLC was formed on October 14, 2020. All intercompany balances and transactions have been eliminated in consolidation.

The unaudited condensed consolidated financial statements have been prepared on the same basis as the audited annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2022, the condensed consolidated statements of operations and comprehensive loss for the three months ended March 31, 2022 and 2021, the condensed consolidated statements of convertible preferred stock and stockholders' equity for the three months ended March 31, 2022 and 2021, and the results of its cash flows for the three months ended March 31, 2022 and 2021. The financial data and other information disclosed in these notes related to the three months ended March 31, 2022 and 2021 are also unaudited. The results for the three months ended March 31, 2022 are not necessarily indicative of results to be expected for the year ending December 31, 2022, any other periods, or any future year period. The Company has evaluated subsequent events through the date the condensed consolidated financial statements were issued.

The accompanying unaudited condensed consolidated financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2021, as filed with the Securities and Exchange Commission (“SEC”) on March 7, 2022.

Segments

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment.

Cash, Cash Equivalents and Restricted Cash

Cash, Cash Equivalents, Restricted Cash and Marketable Securities

The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents include amounts invested in money market accounts.

Restricted cash consists of a money market account which serves as collateral for the Company’s employee corporate credit cards and is classified within other long-term assets on the consolidated balance sheets.

Marketable Securities

The Company's short-term marketable securities have original maturities of less than a year at date of purchase. The Company classifies and accounts for marketable securities as available-for-sale securities, which are carried at their fair values based on the quoted market prices of the securities. Unrealized gains and losses are reported as accumulated other comprehensive loss. Realized gains and losses on available-for-sale securities are included in net loss in the period earned or incurred. As of March 31, 2022 and December 31, 2021, marketable securities consisted of U.S. Treasury bills.

Short-term marketable securities are recorded at their estimated fair value. The Company periodically reviews whether its securities may be other-than-temporarily impaired, including whether or not (i) the Company has the intent to sell the security or (ii) it is more likely than not that the Company will be required to sell the security before its anticipated recovery. If one of these factors is met, the Company will record an impairment loss associated with its impaired investment. The impairment loss will be recorded as a write-down of investments in the condensed consolidated balance sheets and a realized loss within other expense in the condensed consolidated statements of operations.

Leases

The Company determines if an arrangement is or contains a lease at contract inception by assessing whether the arrangement contains an identified asset and whether the lessee has the right to control such asset. Lessees are required to classify leases as either finance or operating leases and to record a right-of-use (“ROU”) asset and a lease liability for all leases with a term greater than 12 months regardless of the lease classification. The lease classification will determine whether the lease expense is recognized based on an effective interest rate method or on a straight-line basis over the term of the lease. The Company determines the initial classification and measurement of its ROU assets and lease liabilities at the lease commencement date and thereafter if modified. For leases with a term greater than 12 months, the Company records the lease liability at the present value of lease payments over the term. The term of the Company’s leases equals the non-cancellable period of the lease, including any rent-free periods provided by the lessor, and also includes options to extend or terminate the lease that the Company is reasonably certain to exercise. The ROU asset equals the carrying amount of the related lease liability, adjusted for any lease payments made prior to lease commencement, any deferred rent upon adoption, and lease incentives provided by the lessor.

The Company has elected, for all classes of underlying assets, not to recognize ROU assets and lease liabilities for leases with a term of 12 months or less. Lease cost for short-term leases is recognized on a straight-line basis over the lease term. The Company’s leases do not provide a readily determinable implicit rate. Therefore, the Company estimates its incremental borrowing rate to discount the lease payments based on information available at lease commencement. The Company determines its incremental borrowing rate based on the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment.

Variable lease payments are expensed as incurred and do not factor into the measurement of the applicable ROU asset or lease liability. Lease payments may be fixed or variable; however, only fixed payments are included in the Company’s lease liability calculation. Lease costs for the Company’s operating leases are recognized on a straight-line basis within operating expenses over the lease term. The Company’s lease agreements may contain non-lease components such as common area maintenance, operating expenses or other costs, which are expensed as incurred for all classes of assets.

Recent Accounting Pronouncements Adopted

In February 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2016-02, Leases (Topic 842) which supersedes FASB ASC Topic 840, Leases (Topic 840) and provides principles for the recognition, measurement, presentation, and disclosure of leases for both lessees and lessors. The Company adopted the new standard effective January 1, 2022 using the modified retrospective transition approach. Upon adoption on January 1, 2022, the Company recognized ROU assets and lease liabilities totaling $15.4 million and $8.9 million, respectively, to reflect the present value of remaining lease payments under existing lease arrangements. The Company applied the modified retrospective transition approach and did not recast prior periods. As permitted by the standard, the Company elected the transition practical expedient package, which, among other things, allows the carryforward of historical lease classifications. The Company’s new accounting policies around leases are described in Leases, above, and in Note 6, Commitments and Contingencies.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. Additionally, the FASB issued ASU No. 2019-04, Codification Improvements to Topic 326, in April 2019 and ASU 2019-05, Financial Instruments — Credit Losses (Topic 326) — Targeted Transition Relief, in May 2019. The amendments affect loans, debt securities, trade receivables, net investments in leases, off-balance-sheet credit exposures, reinsurance receivables, and any other financial assets not excluded from the scope that have the contractual right to receive cash. The Company adopted this standard on January 1, 2022. The adoption of this standard did not have a material impact on the Company’s condensed consolidated financial statements and disclosures.

In December 2019, the FASB issued ASU No. 2019-12. Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The standard simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and also improves consistent application by clarifying and amending existing guidance. The Company adopted this standard on January 1, 2022. The adoption of this standard did not have a material impact on the Company’s condensed consolidated financial statements and disclosures.