Summary of Significant Accounting and Reporting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Consolidation	Consolidation The consolidated financial statements include the accounts of Gentex Corporation and all of its wholly-owned subsidiaries (together the “Company”). All significant intercompany accounts and transactions have been eliminated.
Cash Equivalents	Cash Equivalents Cash equivalents consist of funds invested in bank accounts and money market funds that have daily liquidity.
Allowance For Doubtful Accounts	Allowance For Doubtful Accounts The Company bases its allowances for doubtful accounts related to receivables on historical credit and collections experience, and the specific identification of other potential problems impacting collectability, including the economic climate. Actual collections can differ, requiring adjustments to the allowances. Individual accounts receivable balances are evaluated on a monthly basis, and those balances considered uncollectible are charged to the allowance. Collections of amounts previously written off are recorded as an increase to the allowance.
Investments	ASC 320, Accounting for Certain Investments in Debt and Equity Securities, as amended and interpreted, provides guidance on determining when an investment is other-than-temporarily impaired. The Company reviews its fixed income and equity investment portfolio for any unrealized losses that would be deemed other-than-temporary and require the recognition of an impairment loss in income. If the cost of an investment exceeds its fair value, the Company evaluates, among other factors, general market conditions, the duration and extent to which the fair value is less than cost, and the Company’s intent and ability to hold the investments. Management also considers the type of security, related-industry and sector performance, as well as published investment ratings and analyst reports, to evaluate its portfolio. Once a decline in fair value is determined to be other-than-temporary, an impairment charge is recorded and new cost basis in the investment is established. If market, industry, and/or investee conditions deteriorate, the Company may incur future impairments. Investments The Company follows the provisions of ASC 820, Fair Value Measurements and Disclosures, for its financial assets and liabilities, and for its non-financial assets and liabilities subject to fair value measurements. ASC 820 provides a framework for measuring the fair value of assets and liabilities. This framework is intended to provide increased consistency in how fair value determinations are made under various existing accounting standards that permit, or in some cases, require estimates of fair-market value. This standard also expanded financial statement disclosure requirements about a company’s use of fair-value measurements, including the effect of such measure on earnings. The cost of securities sold is based on the specific identification method. The Company’s common stocks and certain mutual funds are classified as available for sale and are stated at fair value based on quoted market prices, and as such are classified as Level 1 assets. The Company determines the fair value of its government securities, corporate bonds, and certain mutual funds by utilizing monthly valuation statements that are provided by its broker. The broker determines the investment valuation by utilizing the bid price in the market and also refers to third party sources to validate valuations, and as such are classified as Level 2 assets. The Company's certificates of deposit have remaining maturities of less than one year and are classified as available for sale, and are considered as Level 1 assets. These investments are carried at cost, which approximates fair value.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist of cash and cash equivalents, investments, accounts receivable accounts payable, short and long term debt. The Company’s estimate of the fair values of these financial instruments approximates their carrying amounts at December 31, 2017 and 2016.
Inventories	Inventories Inventories include material, direct labor and manufacturing overhead and are valued at the lower of first-in, first-out (FIFO) cost or market.
Plant and Equipment	Plant and Equipment Plant and equipment is stated at cost. Depreciation and amortization are computed for financial reporting purposes using the straight-line method, with estimated useful lives of 7 to 30 years for buildings and improvements, and 3 to 10 years for machinery and equipment.
Impairment or Disposal of Long-Lived Assets	Impairment or Disposal of Long-Lived Assets The Company reviews long-lived assets, including property, plant and equipment and other intangible assets with definite lives, for impairment whenever events or changes in circumstances indicate that the asset’s carrying amount may not be recoverable. The Company conducts its long-lived asset impairment analysis in accordance with ASC 360-10-15, Impairment or Disposal of Long-Lived Assets. ASC 360-10-15 requires the Company to group assets and liabilities at the lowest level for which identifiable cash flows are largely independent of the cash flows of other assets and liabilities and evaluate the asset group against the sum of the undiscounted future cash flows. If the undiscounted cash flows do not indicate the carrying amount of the asset is recoverable, an impairment charge is measured as the amount by which the carrying amount of the asset group exceeds its fair value based on discounted cash flow analysis or appraisals.
Patents	Patents The Company’s policy is to capitalize costs incurred to obtain patents. The cost of patents is amortized over their useful lives. The cost of patents in process is not amortized until issuance. The Company periodically obtains intellectual property rights, in the ordinary course of business, and the cost of the rights are amortized over their useful lives.
Goodwill and Other Intangible Assets	Goodwill and Intangible Assets Goodwill reflects the cost of an acquisition in excess of the fair values assigned to identifiable net assets acquired. The Company reviews goodwill for impairment during the fourth quarter on an annual basis or more frequently if events or changes in circumstances indicate that goodwill might be impaired. The Company performs an impairment review for its automotive reporting unit, which has been determined to be one of the Company’s reportable segments using a fair value method which incorporates management’s judgments and assumptions, as well as incorporates third party valuations. The fair value of a reporting unit refers to the price that would be received to sell the unit as a whole in an orderly transaction between market participants at the measurement date. In estimating the fair value, the Company uses a combination of widely accepted valuation methodologies incorporating certain judgments and assumptions to arrive at the fair value of the reporting unit. The assumptions included in the impairment tests require judgment and changes to these inputs could impact the results of the calculations which could result in an impairment charge in future periods if the carrying amount of the reporting unit exceeds its calculated fair value. Other than management's internal projections of future cash flows, the primary assumptions used in the step 1 and step 2 impairment tests are the weighted-average cost of capital and long-term growth rates. Although the Company's cash flow forecasts are based on assumptions that are considered reasonable by management and consistent with the plans and estimates management is using to operate the underlying business, there are significant judgments in determining the expected future cash flows attributable to a reporting unit. There have been no impairment charges booked currently or in prior periods in which goodwill existed. Indefinite lived intangible assets are also subject to annual impairment testing or more frequently if indicators of impairment are identified. Management judgment and assumptions are required in determining the underlying fair value of the indefinite lived intangible assets. While the Company believes the judgments and assumptions used in determining fair value are reasonable, different assumptions could change the estimated fair values and, therefore, impairment charges could be required, which could be material to the consolidated financial statements. The indefinite lived intangible assets were not impaired as a result of the annual test prepared by management for either period presented.
Revenue Recognition	Revenue Recognition The Company’s revenue is generated from sales of its products. Sales are recognized when the product is shipped and legal title has passed to the customer. The Company does not generate sales from arrangements with multiple deliverables. Effective January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers, using the modified retrospective method. This guidance supersedes nearly all existing revenue recognition guidance under US GAAP. The core principle of the guidance is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The Company has drafted its accounting policy for the new standard based on a detailed review of its business and contracts. Based on the new guidance, the Company expects to continue recognizing revenue at a particular point in time for the majority of its contracts with customers, which is generally when products are either shipped or delivered, and therefore the adoption of the standard did not have a material impact on its consolidated financial statements.
Advertising and Promotional Materials	Advertising and Promotional Materials All advertising and promotional costs are expensed as incurred
Repairs and Maintenance	Repairs and Maintenance Major renewals and improvements of property and equipment are capitalized, and repairs and maintenance are expensed as incurred.
Self-Insurance	Self-Insurance The Company is self-insured for a portion of its risk on workers’ compensation and employee medical costs. The arrangements provide for stop loss insurance to manage the Company’s risk. Operations are charged with the cost of claims reported and an estimate of claims incurred but not reported, based upon historical claims lag information and other data.
Product Warranty	Product Warranty The Company periodically incurs product warranty costs. Any liabilities associated with product warranty are estimated based on known facts and circumstances and are not significant at December 31, 2017, 2016 and 2015. The Company does not offer extended warranties on its products.
Income Taxes	Income Taxes The provision for income taxes is based on the earnings reported in the consolidated financial statements. Deferred income tax assets and liabilities are computed for differences between the financial statement and tax basis of assets and liabilities that will result in taxable or deductible amounts in the future. Such deferred income tax asset and liability computations are based on enacted tax laws and rates.
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) Comprehensive income (loss) reflects the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. For the Company, comprehensive income represents net income adjusted for unrealized gains and losses on certain investments, unrealized gains and losses on certain derivative financial instruments, and foreign currency translation adjustments that are further detailed in Note 9 to the Consolidated Financial Statements.
Foreign Currency Translation	Foreign Currency Translation The financial position and results of operations of the Company’s foreign subsidiaries are measured using the local currency as the functional currency. Assets and liabilities are translated at the exchange rate in effect at year-end. Income statement accounts are translated at the average rate of exchange in effect during the year. The resulting translation adjustment is recorded as a separate component of shareholders’ investment. Gains and losses arising from re-measuring foreign currency transactions into the appropriate currency are included in the determination of net income.
Derivative Financial Instruments	Derivative Financial Instruments The Company accounts for derivative financial instruments in accordance with the guidance provided in ASC Topic 815, Derivatives and Hedging. The guidance requires that all derivative instruments be recognized as assets or liabilities on the consolidated balance sheets and measured at fair value. For derivatives designated as cash flow hedges, fair value changes in the effective portion of the hedging instrument are recognized in accumulated other comprehensive income on the consolidated balance sheets until the forecasted transaction affects earnings of the consolidated entity. Any ineffective portion of the fair value change is recognized in earnings immediately. The Company reported a loss of $.1 million in earnings for the year ended December 31, 2017 as a result of the ineffectiveness of the hedge. As of December 31, 2016, there was no ineffectiveness. The Company seeks to reduce exposure to interest rate fluctuations through the use of an interest rate swap agreement. The Company does not buy and sell such financial instruments for investment or speculative purposes. The Company is exposed to credit loss in the event of nonperformance by the counterparties on derivative contracts. It is the Company’s practice to manage its credit risk on these transactions by dealing with highly rated financial institutions.
Stock-Based Compensation Plans	Stock-Based Compensation Plans The Company accounts for stock-based compensation using the fair value recognition provisions of ASC 718, “Compensation - Stock Compensation.” As described more fully in Note 5, the Company provides compensation benefits under two stock option plans, a restricted stock plan and an employee stock purchase plan.
Estimates	Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Recent Accounting Standards	Recent Accounting Standards In March 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-09, Improvements to Employee Share-Based Payment Accounting, which amends Accounting Standards Codification (ASC) Topic 718, Compensation - Stock Compensation. ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Under previous guidance, excess tax benefits and deficiencies from stock-based compensation arrangements were recorded in equity when the awards vested or were settled. ASU 2016-09 requires prospective recognition of excess tax benefits and deficiencies in the income statement as a component of the income tax provision. In addition, under ASU 2016-09, excess income tax benefits from stock-based compensation arrangements are classified as cash flow from operations, rather than as cash flow from financing activities. ASU 2016-09 also allows for the Company to make an entity-wide accounting policy election to either estimate the number of awards that are expected to vest or account for forfeitures when they occur. The Company will continue to apply its existing entity-wide policy to estimate the number of awards expected to be forfeited. The Company adopted this standard in the first quarter of 2017. Impact to Consolidated Statements of Income One of the more significant impacts of adopting ASU 2016-09 is the required change in how the Company recognizes the excess tax benefits or deficiencies related to share-based compensation. For example, prior to adopting ASU 2016-09 such benefits and deficiencies were credited or charged, respectively, to additional paid-in capital in the Company’s Consolidated Balance Sheets. Under ASU 2016-09, these benefits and deficiencies are recognized as a discrete tax benefit or discrete tax expense, in the Company’s Consolidated Statements of Income. For the years ended December 31, 2017, the Company recognized a net discrete tax benefit of $5.2 million, related to net tax benefits from share-based compensation. ASU 2016-09 requires companies to adopt the amendment related to accounting for benefits and deficiencies on a prospective basis only. As a result, no change has been made to the Consolidated Statements of Income for the years ended December 31, 2016 and 2015 related to the $3.2 million and $1.2 million of net tax benefits the Company recognized as additional paid-in capital during each respective period. Net tax benefits of $3.2 million recognized as additional paid-in-capital during the year ended December 31, 2016 includes gross tax benefits of $4.9 million net of $1.7 million tax expense. Net tax benefits of $1.2 million recognized as additional paid-in-capital during the year ended December 31, 2015 includes gross tax benefits of $1.7 million net of $.5 million tax expense. In consideration of the impact of the adoption of this standard to earnings per share, the total impact of adoption of this standard to the earnings per share calculation was less than $.02 for the year ending December 31, 2017. Impact to Consolidated Statements of Cash Flows In addition to the income tax consequences described above, under ASU 2016-09 all tax benefits related to share-based payments are reported as cash flows from operating activities along with all other income tax cash flows. Previously, tax benefits from share-based payment arrangements were reported as cash flows from financing activities. With respect to the classification of tax benefits on the statement of cash flows, ASU 2016-09 allows companies to elect either a prospective or retrospective application. The Company has elected to apply this classification amendment retrospectively. As a result, the Company elected to reclassify $5.6 million of tax expense previously reported as cash flows from financing activities on the Company’s Consolidated Statement of Cash Flows for the twelve months ended December 31, 2016 as cash flows from operating activities. Effective January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers, using the modified retrospective method. This guidance supersedes nearly all existing revenue recognition guidance under US GAAP. The core principle of the guidance is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The Company has drafted its accounting policy for the new standard based on a detailed review of its business and contracts. Based on the new guidance, the Company continues to recognize revenue at a particular point in time for the majority of its contracts with customers, which is generally when products are either shipped or delivered. Therefore, the adoption of ASC 606 did not have a material impact on the consolidated financial statements. The Company anticipates it will expand its consolidated financial statement disclosures in order to comply with the disclosure requirements of the ASU beginning in the first quarter of 2018. Effective January 1, 2018, the Company adopted ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The standard amends various aspects of the recognition, measurement, presentation, and disclosure of financial instruments. The most significant impact to our consolidated financial statements relates to the recognition and measurement of equity investments at fair value with changes recognized in Net income. The amendment also updates certain presentation and disclosure requirements. The Company will have a cumulative-effect adjustment in the first quarter of 2018 of approximately $7 million as a result of the implementation of this guidance, as a result of the reclassification of the net unrealized gain for available-for-sale securities as of December 31, 2017 from other comprehensive income to retained earnings. The adoption of ASU 2016-01 is expected to increase volatility in net income as changes in the fair value of available-for-sale equity investments and changes in observable prices of equity investments without readily determinable fair values will be recorded in net income. In February 2016, the FASB issued ASU 2016-02, Leases, which provides guidance for lease accounting. The new guidance contained in the ASU stipulates that lessees will need to recognize a right-of-use asset and a lease liability for substantially all leases (other than leases that meet the definition of a short-term lease). The liability will be equal to the present value of lease payments. Treatment in the consolidated statements of earnings will be similar to the current treatment of operating and capital leases. The new guidance is effective on a modified retrospective basis for the Company in the first quarter of its fiscal year ending December 31, 2019. The Company is currently in the process of evaluating the impact of adoption of this standard on its consolidated financial statements.