Basis of Presentation	3 Months Ended
Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	(1) 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, 2017. 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 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 5, '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 requires application on a modified retrospective basis with the earliest period presented. During March 2018, the FASB approved amendments to create an optional transition method that will provide an option to use the effective date of ASC 842, Leases, as the date of initial application of transition. We plan to elect this option. Based on the effective date, this guidance will apply and we will adopt this ASU beginning on January 1, 2019. While we are still in the process of evaluating the effect of adoption on our consolidated financial statements and are currently assessing our leases, we expect the adoption will lead to a material increase in the assets and liabilities recorded on our Condensed Consolidated Balance Sheets. As part of our assessment, we will need to determine the impact of lease extension provisions provided in our facility and vehicle leases, which will impact the amount of the right of use asset and lease liability recorded under the ASU.

(1) 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, 2017. 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

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 5, '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 requires application on a modified retrospective basis with the earliest period presented. During March 2018, the FASB approved amendments to create an optional transition method that will provide an option to use the effective date of ASC 842, Leases, as the date of initial application of transition. We plan to elect this option. Based on the effective date, this guidance will apply and we will adopt this ASU beginning on January 1, 2019. While we are still in the process of evaluating the effect of adoption on our consolidated financial statements and are currently assessing our leases, we expect the adoption will lead to a material increase in the assets and liabilities recorded on our Condensed Consolidated Balance Sheets. As part of our assessment, we will need to determine the impact of lease extension provisions provided in our facility and vehicle leases, which will impact the amount of the right of use asset and lease liability recorded under the ASU.