Business Overview and Summary of Significant Accounting Policies	12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]
Business Overview and Summary of Significant Accounting Policies	Note 1. Business Overview and Summary of Significant Accounting Policies Business Overview Fastenal is a leader in the wholesale distribution of industrial and construction supplies operating a branch-based business (with an increasing number of Onsite locations). Collectively we refer to our branches and Onsite locations as in-market locations. We have approximately 3,000 in-market locations located primarily in North America. 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 and Accounts Receivable Net sales include products, services, shipping and handling charges, and lease fees billed, net of any related sales incentives, and net of an estimate for product returns. We recognize revenue when persuasive evidence of an arrangement exists, title and risk of ownership have passed, the sales price is fixed or determinable, and collectibility is reasonably assured. These criteria are met at the time the product is shipped to or picked up by the customer. We recognize services at the time the service is completed and the product is provided to the customer. We recognize revenue for shipping and handling charges at the time the products are shipped to or picked up by the customer. We recognize revenue for lease fees on a straight-line basis over the corresponding lease term. We estimate product returns based on historical return rates. 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. Sales taxes (and value added taxes in foreign jurisdictions) collected from customers and remitted to governmental authorities are accounted for on a net basis and therefore are excluded from net sales. 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 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 market. 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. There were no impairments recorded during any of the three years reported in these consolidated financial statements. 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 Accounting Pronouncements Effective January 1, 2017, we adopted the FASB 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 Consolidated Statements of Cash Flows. As a result of the adoption, on a prospective basis, for the year ended December 31, 2017, we recognized $1.8 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 Consolidated Statements of Cash Flows for the years ended December 31, 2016 and December 31, 2015, which resulted in a reclassification of excess tax benefits from stock-based compensation of $5.9 and $3.4, respectively, offsetting cash flows used in financing activities to cash flows provided by 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. On December 22, 2017, the Securities and Exchange Commission ('SEC') staff issued Staff Accounting Bulletin No. 118 ('SAB 118') to address the application of U.S. GAAP related to the enactment of the comprehensive tax legislation, commonly referred to as the Tax Cut and Jobs Act (the 'Tax Act'). This guidance was adopted in the fourth quarter of 2017. Additional information regarding our adoption of this guidance is contained in Note 7. 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 deferred 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 was 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 deferred 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 modified retrospective 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. We have completed the process of evaluating the effect of the adoption and determined there were no changes required to our reported revenues as a result of the adoption. The majority of our revenue arrangements generally consist of a single performance obligation to transfer promised goods or services. Based on our evaluation process and review of our contracts with customers, the timing and amount of revenue recognized based on ASU 2015-14 is consistent with our revenue recognition policy under previous guidance. We adopted the new standard effective January 1, 2018, using the modified retrospective approach, and will expand our consolidated financial statement disclosures in order to comply with the ASU. We have determined the adoption of ASU 2015-14 will not have a material impact on our results of operations, cash flows, or financial position. 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 will 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 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.

Note 1. Business Overview and Summary of Significant Accounting Policies

Business Overview

Fastenal is a leader in the wholesale distribution of industrial and construction supplies operating a branch-based business (with an increasing number of Onsite locations). Collectively we refer to our branches and Onsite locations as in-market locations. We have approximately 3,000 in-market locations located primarily in North America.

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 and Accounts Receivable

Net sales include products, services, shipping and handling charges, and lease fees billed, net of any related sales incentives, and net of an estimate for product returns. We recognize revenue when persuasive evidence of an arrangement exists, title and risk of ownership have passed, the sales price is fixed or determinable, and collectibility is reasonably assured. These criteria are met at the time the product is shipped to or picked up by the customer. We recognize services at the time the service is completed and the product is provided to the customer. We recognize revenue for shipping and handling charges at the time the products are shipped to or picked up by the customer. We recognize revenue for lease fees on a straight-line basis over the corresponding lease term. We estimate product returns based on historical return rates. 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. Sales taxes (and value added taxes in foreign jurisdictions) collected from customers and remitted to governmental authorities are accounted for on a net basis and therefore are excluded from net sales.

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 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 market.

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. There were no impairments recorded during any of the three years reported in these consolidated financial statements.

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 Accounting Pronouncements

Effective January 1, 2017, we adopted the FASB 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 Consolidated Statements of Cash Flows. As a result of the adoption, on a prospective basis, for the year ended December 31, 2017, we recognized $1.8 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 Consolidated Statements of Cash Flows for the years ended December 31, 2016 and December 31, 2015, which resulted in a reclassification of excess tax benefits from stock-based compensation of $5.9 and $3.4, respectively, offsetting cash flows used in financing activities to cash flows provided by 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.

On December 22, 2017, the Securities and Exchange Commission ('SEC') staff issued Staff Accounting Bulletin No. 118 ('SAB 118') to address the application of U.S. GAAP related to the enactment of the comprehensive tax legislation, commonly referred to as the Tax Cut and Jobs Act (the 'Tax Act'). This guidance was adopted in the fourth quarter of 2017. Additional information regarding our adoption of this guidance is contained in Note 7.

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 deferred 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 was 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 deferred 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 modified retrospective 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. We have completed the process of evaluating the effect of the adoption and determined there were no changes required to our reported revenues as a result of the adoption. The majority of our revenue arrangements generally consist of a single performance obligation to transfer promised goods or services. Based on our evaluation process and review of our contracts with customers, the timing and amount of revenue recognized based on ASU 2015-14 is consistent with our revenue recognition policy under previous guidance. We adopted the new standard effective January 1, 2018, using the modified retrospective approach, and will expand our consolidated financial statement disclosures in order to comply with the ASU. We have determined the adoption of ASU 2015-14 will not have a material impact on our results of operations, cash flows, or financial position.

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 will 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 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.