Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2020
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation— The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 GmbH”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated.
Unaudited Interim Financial Statements	Unaudited Interim Financial Statements— The condensed consolidated balance sheet at December 31, 2019 was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP"). The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the SEC for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the SEC on March 12, 2020. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020.
Use of Estimates	Use of Estimates— The preparation of the Company’s condensed consolidated 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 condensed consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and the constraint of variable consideration from contracts with customers. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. The Company anticipates that the COVID-19 pandemic will have an impact on the clinical development timelines for certain of its programs. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2020 and December 31, 2019.
Goodwill	Goodwill— Goodwill is tested for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit. The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than its carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit. Given the ongoing COVID-19 pandemic and associated volatility reflected in the global financial markets and uncertain impact on the Company’s operations, management conducted an interim test of its goodwill as of March 31, 2020. The Company determined that goodwill was not impaired as of March 31, 2020.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 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”). The amendments in this update align 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 accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The new standard was effective beginning January 1, 2020 and was adopted by the Company on that date. The adoption of ASU 2018-15 did not have an impact on the Company's consolidated financial statements and related disclosures. In August 2018, the 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 removes, adds and modifies certain disclosure requirements for fair value measurements in Topic 820. The Company will no longer be required to disclose the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy as well as the valuation processes of Level 3 fair value measurements. The Company will be required to provide additional disclosure related to the changes in unrealized gains and losses included in other comprehensive loss for recurring Level 3 fair value measurements and the range and weighted average of assumptions used to develop significant unobservable inputs for Level 3 fair value measurements. ASU 2018-13 is effective for the Company on January 1, 2020 and was adopted on that date. The adoption of ASU 2018-13 did not have a significant impact on the Company’s consolidated financial statements and related disclosures. Recently Issued Accounting Standards Not Yet Adopted In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes (“ASU 2019-12”). ASU 2019-12 simplifies the accounting for income taxes, including the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU 2019-12 is effective for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. The Company does not anticipate that the adoption of ASU 2019-12 will have a material impact on its consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13"), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. In accordance with ASU 2019-10, Financial Instruments-Credit Losses (Topic 326), Derivative and Hedging (Topic 815), and Leases (Topic 842)- Effective Dates, as the Company meets the definition of a "smaller reporting company", the Company has elected to defer the adoption of ASU 2016-13 until January 1, 2023. The Company expects that the adoption of ASU 2016-13 may accelerate the timing and could increase the level of credit loss expense in the consolidated statement of operations and will likely require an increased level of disclosure in the notes to the consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

Principles of Consolidation— The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, including X4 Pharmaceuticals (Austria) GmbH, which is incorporated in Vienna, Austria (“X4 GmbH”), and X4 Therapeutics, Inc. All significant intercompany accounts and transactions have been eliminated.

Unaudited Interim Financial Statements

Unaudited Interim Financial Statements— The condensed consolidated balance sheet at December 31, 2019 was derived from audited financial statements but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP"). The accompanying condensed consolidated financial statements are unaudited. The accompanying unaudited interim consolidated financial statements have been prepared by the Company pursuant to the rules and regulations of the SEC for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto for the year ended December 31, 2019 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the SEC on March 12, 2020. In the opinion of management, all adjustments, consisting only of normal recurring adjustments as necessary, for the fair statement of the Company’s condensed financial position, condensed results of its operations and cash flows have been made. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2020.

Use of Estimates

Use of Estimates— The preparation of the Company’s condensed consolidated 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 condensed consolidated financial statements, and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual of research and development expenses, the impairment or lack of impairment of long-lived assets including operating lease right-of-use assets and goodwill, and the constraint of variable consideration from contracts with customers. The Company bases its estimates on historical experience, known trends and other market-specific or other relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates when there are changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. The Company anticipates that the COVID-19 pandemic will have an impact on the clinical development timelines for certain of its programs. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s financial statements.

Cash and Cash Equivalents

Cash and Cash Equivalents— The Company considers all highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents consisted of money market funds as of March 31, 2020 and December 31, 2019.

Goodwill

Goodwill— Goodwill is tested for impairment at the reporting unit level annually in the fourth quarter, or more frequently when events or changes in circumstances indicate that the asset might be impaired. Examples of such events or circumstances include, but are not limited to, a significant adverse change in legal or business climate, an adverse regulatory action or unanticipated competition. The Company has determined that it operates in a single operating segment and has a single reporting unit.

The Company assesses qualitative factors to determine whether the existence of events or circumstances would indicate that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. If after assessing the totality of events or circumstances, the Company were to determine that it is more likely than not that the fair value of the reporting unit is less than its carrying amount, then the Company would perform an interim quantitative impairment test, whereby the Company compares the fair value of the reporting unit to its carrying value. If the fair value of the reporting unit exceeds the carrying value of its net assets, goodwill is not impaired, and no further testing is required. If the fair value of the reporting unit is less than its carrying value, the Company measures the amount of impairment loss, if any, as the excess of the carrying value over the fair value of the reporting unit.

Given the ongoing COVID-19 pandemic and associated volatility reflected in the global financial markets and uncertain impact on the Company’s operations, management conducted an interim test of its goodwill as of March 31, 2020. The Company determined that goodwill was not impaired as of March 31, 2020.

Recently Adopted Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 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”). The amendments in this update align 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 accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The new standard was effective beginning January 1, 2020 and was adopted by the Company on that date. The adoption of ASU 2018-15 did not have an impact on the Company's consolidated financial statements and related disclosures.

In August 2018, the 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 removes, adds and modifies certain

disclosure requirements for fair value measurements in Topic 820. The Company will no longer be required to disclose the

amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy as well as the valuation processes of Level 3 fair value measurements. The Company will be required to provide additional disclosure related to the changes in

unrealized gains and losses included in other comprehensive loss for recurring Level 3 fair value measurements and the range and weighted average of assumptions used to develop significant unobservable inputs for Level 3 fair value measurements. ASU 2018-13 is effective for the Company on January 1, 2020 and was adopted on that date. The adoption of ASU 2018-13 did not have a significant impact on the Company’s consolidated financial statements and related disclosures.

Recently Issued Accounting Standards Not Yet Adopted

In December 2019, the FASB issued ASU 2019-12, Simplifying the Accounting for Income Taxes (“ASU 2019-12”). ASU

2019-12 simplifies the accounting for income taxes, including the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU 2019-12 is effective for fiscal years beginning after December 15, 2020, including interim periods within those fiscal years. The Company does not anticipate that the adoption of ASU 2019-12 will have a material impact on its consolidated financial statements and related disclosures.

In June 2016, the FASB issued ASU 2016-13, Credit Losses (Topic 326) Measurement of Credit Losses on Financial Instruments (“ASU 2016-13"), as amended. ASU 2016-13 requires that financial assets measured at amortized cost, such as trade receivables, be presented net of expected credit losses, which may be estimated based on relevant information such as historical experience, current conditions, and future expectation for each pool of similar financial asset. The new guidance requires enhanced disclosures related to trade receivables and associated credit losses. In accordance with ASU 2019-10, Financial Instruments-Credit Losses (Topic 326), Derivative and Hedging (Topic 815), and Leases (Topic 842)- Effective Dates, as the Company meets the definition of a "smaller reporting company", the Company has elected to defer the adoption of ASU 2016-13 until January 1, 2023. The Company expects that the adoption of ASU 2016-13 may accelerate the timing and could increase the level of credit loss expense in the consolidated statement of operations and will likely require an increased level of disclosure in the notes to the consolidated financial statements.