Business Overview and Summary of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	The consolidated financial statements include the accounts of Fastenal Company and its subsidiaries (collectively referred to as 'Fastenal' or by terms such as 'we', 'our', or 'us'). All material intercompany balances and transactions have been eliminated in consolidation.
Revenue Recognition	Net sales include products and shipping and handling charges, net of estimates for product returns and any related sales incentives. Revenue is measured as the amount of consideration we expect to receive in exchange for transferring products. All revenue is recognized when we satisfy our performance obligations under the contract. We recognize revenue by transferring the promised products to the customer, with the majority of revenue recognized at the point in time the customer obtains control of the products. We recognize revenue for shipping and handling charges at the time the products are delivered to or picked up by the customer. We estimate product returns based on historical return rates. Using probability assessments, we estimate sales incentives expected to be paid over the term of the contract. The majority of our contracts have a single performance obligation and are short term in nature. Sales taxes and value added taxes in foreign jurisdictions that are collected from customers and remitted to governmental authorities are accounted for on a net basis and therefore are excluded from net sales.
Accounts Receivable	Credit is extended based upon an evaluation of the customer's financial condition. Accounts receivable are stated at their estimated net realizable value. The allowance for doubtful accounts is based on an analysis of customer accounts and our historical experience with accounts receivable write-offs.
Foreign Currency Translation and Transactions	The functional currency of our foreign operations is typically the applicable local currency. The functional currency is translated into United States dollars for balance sheet accounts, except retained earnings, using current exchange rates as of the balance sheet date, for retained earnings at historical exchange rates, and for revenue and expense accounts using a weighted average exchange rate during the applicable period. The translation adjustments are deferred as a separate component of stockholders' equity captioned accumulated other comprehensive income (loss). Gains or losses resulting from transactions denominated in foreign currencies are included in cost of sales or operating and administrative expenses.
Cash and Cash Equivalents	We consider all investments purchased with original maturities of three months or less to be cash equivalents.
Inventories	Inventories, consisting of finished goods merchandise held for resale, are stated at the lower of cost (first in, first out method) or net realizable value. We establish a reserve for excess, slow-moving, and obsolete inventory that is equal to the difference between the cost and estimated net realizable value for that inventory. These reserves are based on a review and comparison of the current inventory levels to projected and historical sales of inventory.
Property and Equipment	Property and equipment are stated at cost. Depreciation on property and equipment is provided for using the straight-line method over the anticipated economic useful lives of the related property. Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If circumstances require a long-lived asset or asset group be tested for possible impairment, we first compare undiscounted cash flows expected to be generated by the asset or asset group to its carrying value. If the carrying value of the long-lived asset or asset group is not recoverable on an undiscounted cash flow basis, an impairment is recognized to the extent the carrying value exceeds its fair value. Fair value is determined through various valuation techniques including discounted cash flow models, quoted market values, and third-party independent appraisals, as considered necessary.
Leases	We lease space under operating leases for certain distribution centers, branches, and manufacturing locations. These leases do not have significant rent escalation holidays, concessions, leasehold improvement incentives, or other build-out clauses. Any such terms are recognized as rent expense over the term of the lease. Further, the leases do not contain contingent rent provisions. Leasehold improvements on operating leases are amortized over their estimated service lives on a straight-line basis, or the remaining lease term, whichever is shorter. We lease certain semi-tractors, pick-ups, and equipment under operating leases.
Other Long-Lived Assets	Other assets consist of prepaid deposits, goodwill, and other definite-lived intangible assets. Goodwill represents the excess of the purchase price over the fair value of net assets acquired. Goodwill is reviewed for impairment annually. The identifiable intangible assets are amortized on a straight-line basis over their estimated life.
Accounting Estimates	The preparation of the consolidated financial statements in conformity with U.S. generally accepted accounting principles (GAAP) requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities, revenues and expenses, and the disclosure of contingent liabilities. Actual results could differ from those estimates.
Insurance Reserves	We are self-insured for certain losses relating to workers' compensation, automobile, health, and general liability costs. Specific stop-loss coverage is provided for catastrophic claims in order to limit exposure to significant claims. Self-insurance liabilities are based on our estimate of reported claims and claims incurred but not yet reported.
Product Warranties	We offer a basic limited warranty for certain of our products. The specific terms and conditions of those warranties vary depending upon the product sold. We typically recoup these costs through product warranties we hold with the original equipment manufacturers. Our warranty expense has historically been minimal.
Stock-Based Compensation	We estimate the value of stock option grants using a Black-Scholes valuation model. Stock-based compensation expense is recognized on a straight-line basis over the vesting period. Our stock-based compensation expense is recorded in operating and administrative expenses.
Income Taxes	We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. We recognize the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. We record interest and penalties related to unrecognized tax benefits in income tax expense.
Earnings Per Share	Basic net earnings per share is calculated using net earnings available to common stockholders divided by the weighted average number of shares of common stock outstanding during the year. Diluted net earnings per share is similar to basic net earnings per share except that the weighted average number of shares of common stock outstanding includes the incremental shares assumed to be issued upon the exercise of stock options considered to be 'in-the-money' (i.e., when the market price of our stock is greater than the exercise price of our outstanding stock options).
Segment Reporting	We have determined that for our North American operations we meet the aggregation criteria outlined in the accounting standards as our various operations have similar (1) economic characteristics, (2) products and services, (3) customers, (4) distribution channels, and (5) regulatory environments. Considering the insignificance of our operations outside of North America, we report as a single business segment.
Recently Adopted and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements Effective January 1, 2018, we adopted the Financial Accounting Standards Board ('FASB') Accounting Standards Update ('ASU') 2014-09, Revenue from Contracts with Customers (Topic 606), and ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of Effective Date, which deferred the effective date of ASU 2014-09 by one year. ASU 2014-09 supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, and is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. It also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue, cash flows arising from customer contracts, including significant judgments and changes in judgments, and assets recognized from costs incurred to obtain or fulfill a contract. The adoption of ASU 2014-09, using the modified retrospective approach, had no significant impact on our results of operations, cash flows, or financial position. Revenue continues to be recognized at a point in time for our product sales when products are delivered to or picked up by the customer and revenue for shipping and handling charges continues to be recognized when products are delivered to or picked up by the customer. We continue to reduce revenue for estimates of sales incentives based on probability estimates and for product returns based on historical return rates. Additional information and disclosures required by this new standard are contained in Note 2, 'Revenue'. In March 2018, we adopted FASB ASU 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118, which updates the income tax accounting in U.S. GAAP to reflect the Securities and Exchange Commission ('SEC') interpretive guidance released on December 22, 2017, when the Tax Cuts and Jobs Act (the 'Tax Act') was signed into law. Additional information regarding the adoption of this standard is contained in Note 7, 'Income Taxes'. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases, which introduces the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous guidance. The update is effective for annual reporting periods beginning after December 15, 2018, including interim periods within those reporting periods, with early adoption permitted. The original guidance required application on a modified retrospective basis with the earliest period presented. In August 2018, the FASB issued ASU 2018-11, Targeted Improvements to ASC 842, which includes an option to not restate comparative periods in transition and elect to use the effective date of ASC 842, Leases, as the date of initial application of transition. Based on the effective date, this guidance will apply and we will adopt this ASU beginning on January 1, 2019 and plan to elect the transition option provided under ASU 2018-11. We expect this standard will have a material effect on our Consolidated Balance Sheets with the recognition of new right of use assets and lease liabilities for all operating leases, except pick-up truck leases where we expect to elect the short-term lease recognition exemption, as these leases typically have a non-cancelable lease term of approximately one year. Upon adoption, we estimate both assets and liabilities on our Consolidated Balance Sheets will increase by approximately $250.0. Changes in our lease population or changes in incremental borrowing rates may alter this estimate. We will expand our consolidated financial statement disclosures upon adoption of this standard.