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Stockholders' Equity - Additional Information (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended 0 Months Ended
Dec. 31, 2012
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 14, 2015
Schedule of Shareholder's Equity [Line Items]          
Dividends paid per share   $ 1.00us-gaap_CommonStockDividendsPerShareCashPaid $ 0.80us-gaap_CommonStockDividendsPerShareCashPaid $ 1.24us-gaap_CommonStockDividendsPerShareCashPaid  
Dividend paid per share, supplemental $ 0.50fast_CommonStockSupplementalDividendPerShareCashPaid        
Options vesting and exercisable period, maximum   8 years      
Total intrinsic value of stock options exercised   $ 7,466us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 9,925us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 34,424us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue  
Total unrecognized stock-based compensation expense   15,908us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized      
Weighted average period over which total unrecognized stock-based compensation expense will be recognized   4 years 7 months 6 days      
Total grant date fair value of options vested   7,287fast_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedFairValue 3,508fast_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedFairValue 3,866fast_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedFairValue  
Total stock-based compensation expense for the Fastenal stock option plan   $ 7,039us-gaap_ShareBasedCompensation $ 5,400us-gaap_ShareBasedCompensation $ 4,800us-gaap_ShareBasedCompensation  
Options termination period   9 years      
Subsequent Event [Member]          
Schedule of Shareholder's Equity [Line Items]          
Dividend declared date         Jan. 14, 2015
Dividend declared per share         $ 0.28us-gaap_CommonStockDividendsPerShareDeclared
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Dividends payment date         Feb. 27, 2015
Dividends payable record date         Jan. 30, 2015
Common stock purchase authorization date         Jan. 14, 2015
Common stock purchase authorized shares         2,000,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
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