Basis of Presentation (Policies)	6 Months Ended
Jun. 30, 2017
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, 2016. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included.
Recently Adopted and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements Effective January 1, 2017, we adopted the Financial Accounting Standards Board ('FASB') Accounting Standards Update ('ASU') 2016-09, Improvements to Employee Share-Based Payment Accounting. The standard simplifies several aspects of the accounting for employee share-based payment transactions, including accounting for income taxes, forfeitures, and statutory withholding requirements, as well as classification in the Condensed Consolidated Statements of Cash Flows. As a result of the adoption, on a prospective basis, for the six and three month periods ended June 30, 2017, we recognized $0.5 and $0.0, respectively, of excess tax benefits from stock-based compensation as a discrete item in our income tax expense. Historically, these amounts were recorded as additional paid-in capital. Upon adoption, we elected to apply the change retrospectively to our Condensed Consolidated Statement of Cash Flows for the six months ended June 30, 2016, which resulted in a reclassification of excess tax benefits from stock-based compensation of $5.5 from cash flows from financing activities to cash flows from operating activities. We elected not to change our policy on accounting for forfeitures and will continue to estimate a requisite forfeiture rate. Additional amendments to the accounting for income taxes and minimum statutory withholding requirements had no impact on our results of operations. Recently Issued Accounting Pronouncements In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which defers the effective date of ASU 2014-09 for all entities by one year. This update is effective for public business entities for annual reporting periods beginning after December 15, 2017, including interim periods within those reporting periods. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. ASU 2014-09 was to become effective for us beginning January 2017; however, ASU 2015-14 defers our effective date until January 2018, which is when we plan to adopt this standard. The ASU permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the cumulative catch-up transition method). The ASU also requires expanded disclosures relating to the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Additionally, qualitative and quantitative disclosures are required for customer contracts, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. While we are still in the process of evaluating the effect of adoption on our consolidated financial statements and are currently assessing our contracts with customers, we do not currently expect a material impact on our results of operations, cash flows or financial position. We anticipate we will expand our consolidated financial statement disclosures in order to comply with the ASU. We have not yet decided on our transition method upon adoption, but plan to select a transition method in the third quarter of 2017. 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 guidance will be applied on a modified retrospective basis with the earliest period presented. Based on the effective date, this guidance would apply beginning January 2019 which is when we plan to adopt this ASU. 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.

Recently Adopted Accounting Pronouncements

Effective January 1, 2017, we adopted the Financial Accounting Standards Board ('FASB') Accounting Standards Update ('ASU') 2016-09, Improvements to Employee Share-Based Payment Accounting. The standard simplifies several aspects of the accounting for employee share-based payment transactions, including accounting for income taxes, forfeitures, and statutory withholding requirements, as well as classification in the Condensed Consolidated Statements of Cash Flows. As a result of the adoption, on a prospective basis, for the six and three month periods ended June 30, 2017, we recognized $0.5 and $0.0, respectively, of excess tax benefits from stock-based compensation as a discrete item in our income tax expense. Historically, these amounts were recorded as additional paid-in capital. Upon adoption, we elected to apply the change retrospectively to our Condensed Consolidated Statement of Cash Flows for the six months ended June 30, 2016, which resulted in a reclassification of excess tax benefits from stock-based compensation of $5.5 from cash flows from financing activities to cash flows from operating activities. We elected not to change our policy on accounting for forfeitures and will continue to estimate a requisite forfeiture rate. Additional amendments to the accounting for income taxes and minimum statutory withholding requirements had no impact on our results of operations.

Recently Issued Accounting Pronouncements

In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which defers the effective date of ASU 2014-09 for all entities by one year. This update is effective for public business entities for annual reporting periods beginning after December 15, 2017, including interim periods within those reporting periods. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, including interim reporting periods within that reporting period. ASU 2014-09 was to become effective for us beginning January 2017; however, ASU 2015-14 defers our effective date until January 2018, which is when we plan to adopt this standard. The ASU permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the cumulative catch-up transition method). The ASU also requires expanded disclosures relating to the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Additionally, qualitative and quantitative disclosures are required for customer contracts, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. While we are still in the process of evaluating the effect of adoption on our consolidated financial statements and are currently assessing our contracts with customers, we do not currently expect a material impact on our results of operations, cash flows or financial position. We anticipate we will expand our consolidated financial statement disclosures in order to comply with the ASU. We have not yet decided on our transition method upon adoption, but plan to select a transition method in the third quarter of 2017.

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 guidance will be applied on a modified retrospective basis with the earliest period presented. Based on the effective date, this guidance would apply beginning January 2019 which is when we plan to adopt this ASU. 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.