Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: On October 1, 2020, the Company consummated the Business Combination which was accounted for as a reverse recapitalization with Legacy Hyliion being deemed the accounting acquirer in the Business Combination based on an analysis of the criteria outlined in Accounting Standards Codification (“ASC”) 805. Accordingly, for accounting purposes, the Business Combination was treated as the equivalent of Legacy Hyliion issuing stock for the net assets of Tortoise, accompanied by a recapitalization. The net assets of Tortoise are stated at historical cost, with no goodwill or other intangible assets recorded. While Tortoise was the legal acquirer in the Business Combination, because Legacy Hyliion was deemed the accounting acquirer, the historical financial statements of Legacy Hyliion became the historical financial statements of the combined company, upon the consummation of the Business Combination. As a result, the financial statements included in this report reflect (i) the historical operating results of Legacy Hyliion prior to the Business Combination; (ii) the combined results of Tortoise and Legacy Hyliion following the closing of the Business Combination; (iii) the assets and liabilities of Legacy Hyliion at their historical cost; and (iv) the Company’s equity structure for all periods presented. For more details on the reverse recapitalization, see Note 3 to the Company’s Consolidated Financial Statements as presented in its Annual Report, as amended on Form 10-K/A for the year ended December 31, 2020 which was filed with the Securities and Exchange Commission (“SEC”) on May 17, 2021 (the “2020 Amended Annual Report”). As a result of the reverse recapitalization, all references to numbers of common shares and per common share data for 2020 in these condensed consolidated financial statements and related notes have been retroactively adjusted to account for the effect of the reverse recapitalization. These condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary. Intercompany accounts and transactions have been eliminated in consolidation. These interim condensed consolidated statements have been prepared pursuant to the rules and regulations of the SEC, which permit reduced disclosure for interim periods. The Consolidated Balance Sheet as of December 31, 2020 was derived from audited financial statements for the fiscal year then ended, but does not include all necessary disclosures required by accounting principles generally accepted in the United States of America (“GAAP”) with respect to annual financial statements. In the opinion of management, these unaudited condensed consolidated financial statements include all recurring adjustments and normal accruals necessary for a fair presentation of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These condensed consolidated financial statements and accompanying notes should be read in conjunction with the Company’s 2020 Amended Annual Report. Results for interim periods are not necessarily indicative of the results to be expected for a full fiscal year or for any future period.
Emerging Growth Company	Emerging Growth Company: Section 102(b)(1) of the Jumpstart Our Business Startups Act (“JOBS Act”) exempts emerging growth companies from being required to comply with new or revised financial accounting standards until private companies (that is, those that have not had a registration statement under the Securities Act of 1933, as amended (the “Securities Act”) declared effective or do not have a class of securities registered under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) are required to comply with the new or revised financial accounting standards. The JOBS Act provides that a company can elect to opt out of the extended transition period and comply with the requirements that apply to non-emerging growth companies but any such election to opt out is irrevocable. The Company has elected not to opt out of such extended transition period which means that when a standard is issued or revised and it has different application dates for public or private companies, the Company, as an emerging growth company, can adopt the new or revised standard at the time private companies adopt the new or revised standard, until such time the Company is no longer considered to be an emerging growth company. As of June 30, 2021, the last business day of our most recently completed second fiscal quarter, the market value of our common stock that was held by non-affiliates was greater than $700 million. As a result, we will become a large accelerated filer and will no longer qualify as an emerging growth company on December 31, 2021, the end of our current fiscal year. Accordingly, at that time we will no longer qualify for the provisions of the JOBS Act that allow companies to adopt new or revised accounting standards when required by private company accounting standards. We have not elected to defer adoption of any new or revised accounting standards under the provisions of the JOBS Act.
Use of estimates and uncertainty of the coronavirus pandemic	Use of Estimates and Uncertainty of the Coronavirus Pandemic: The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the balance sheet date, as well as reported amounts of expenses during the reporting period. The Company’s most significant estimates and judgments involve valuation of share-based compensation, including the fair value of common stock prior to the Business Combination, and the valuation of the convertible notes payable derivative liability. Management bases its estimates on historical experience and on various other assumptions believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results could differ from those estimates, and such differences could be material to the Company’s condensed consolidated financial statements. On January 30, 2020, the World Health Organization declared the coronavirus outbreak a “Public Health Emergency of International Concern” and on March 11, 2020, declared the coronavirus outbreak a pandemic. In mid-March 2020, U.S. state governors, local officials and leaders outside of the U.S. began ordering various “shelter-in-place” orders, which have had various impacts on the U.S. and global economies. This may require greater use of estimates and assumptions in the preparation of the unaudited condensed consolidated financial statements. As the coronavirus pandemic continues to evolve, the Company believes the extent of the impact to its businesses, operating results, cash flows, liquidity and financial condition will be primarily driven by the severity and duration of the coronavirus pandemic, the pandemic’s impact on the U.S. and global economies and the timing, scope, and effectiveness of federal, state and local governmental responses to the pandemic. Those primary drivers are beyond the Company’s knowledge and control, and as a result, at this time the Company is unable to predict the cumulative impact, both in terms of severity and duration, that the coronavirus pandemic will have on its business, operating results, cash flows, liquidity, and financial condition, but it could be material if the current circumstances continue to exist for a prolonged period. Although the Company has made its best estimates based upon current information, actual results could materially differ from the estimates and assumptions developed by management. If so, the Company may be subject to future impairment charges as well as changes to recorded reserves and valuations.
Restricted Cash	Restricted Cash: On July 2, 2021, the Company provided its corporate headquarters lessor with a letter of credit for $0.7 million to secure the performance of lease obligations. The Company made a restricted cash deposit for its obligation to pay any draws on the letter of credit by the lessor.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements: In December 2019, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which is intended to simplify various aspects related to accounting for income taxes. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2020. The Company adopted ASU 2019-12 on January 1, 2021. However, there is no impact to the Company as a result of the adoption in the current quarter, nor is there an expected impact to the Company for the remainder of the year.