Basis of Presentation (Policies)	3 Months Ended
Basis of Presentation (Policies)	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The accompanying unaudited condensed consolidated financial statements of Fastenal Company and subsidiaries (collectively referred to as the company, Fastenal, or by terms such as we, our, or us) have been prepared in accordance with U.S. generally accepted accounting principles ('GAAP') for interim financial information. They do not include all information and footnotes required by U.S. GAAP for complete financial statements. However, except as described herein, there has been no material change in the information disclosed in the Notes to Consolidated Financial Statements included in our consolidated financial statements as of and for the year ended December 31, 2018. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Effective January 1, 2019, we adopted the Financial Accounting Standards Board ('FASB') Accounting Standards Update ('ASU') 2016-02, Leases, which requires the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous guidance. 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 included 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, which we elected. As a result of the adoption of ASC 842 on January 1, 2019, we recorded both operating lease right-of-use ('ROU') assets of $227.5 and lease liabilities of $228.3. The adoption of ASC 842 had an immaterial impact on our Condensed Consolidated Statement of Earnings and Condensed Consolidated Statement of Cash Flows for the three-month period ended March 31, 2019. In addition, we elected the package of practical expedients permitted under the transition guidance within the new standard which allowed us to carry forward the historical lease classification. Additional information and disclosures required by this new standard are contained in Note 5, 'Operating Leases'.
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.
Lease and Nonlease Components	Many of our leases include both lease (e.g., fixed payments including rent, taxes, and insurance costs) and non-lease components (e.g., common-area or other maintenance costs) which are accounted for as a single lease component as we have elected the practical expedient to group lease and non-lease components for all leases.
Short-term Leases	Our pick-up truck leases typically have a non-cancelable lease term of less than one year and therefore, we have elected the practical expedient to exclude these short-term leases from our ROU assets and lease liabilities.
Lessee, Leases	Most leases include one or more options to renew. The exercise of lease renewal options is typically at our sole discretion; therefore, the majority of renewals to extend the lease terms are not included in our ROU assets and lease liabilities as they are not reasonably certain of exercise. We regularly evaluate the renewal options and when they are reasonably certain of exercise, we include the renewal period in our lease term. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at the lease commencement date in determining the present value of the lease payments. We have a centrally managed treasury function; therefore, based on the applicable lease terms and the current economic environment, we apply a portfolio approach for determining the incremental borrowing rate. To the extent our fleet contains vehicles we estimate will settle at a gain, such gains on these vehicles will be recognized when we sell the vehicle.