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Accounting Policies (Concentration Risk) (Details) (Sales revenue, Services, Maximum)
12 Months Ended
Dec. 31, 2014
Sales revenue | Services | Maximum
 
Concentration Risk [Line Items]  
Service revenue, less than 1.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ProductOrServiceAxis
= graybar_ServicesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember