Revenue - Percentages of Sales by Product Line (Details) - Product Line - Sales	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Percentage of Sales by Product Line [Line Items]
Percentage of sales	100.00%	100.00%	100.00%	100.00%
Fasteners
Percentage of Sales by Product Line [Line Items]
Percentage of sales	34.70%	35.60%	35.10%	35.80%
Tools
Percentage of Sales by Product Line [Line Items]
Percentage of sales	10.10%	10.30%	10.10%	10.10%
Cutting tools
Percentage of Sales by Product Line [Line Items]
Percentage of sales	5.70%	5.80%	5.70%	5.80%
Hydraulics & pneumatics
Percentage of Sales by Product Line [Line Items]
Percentage of sales	6.70%	6.70%	6.90%	6.80%
Material handling
Percentage of Sales by Product Line [Line Items]
Percentage of sales	5.70%	5.80%	5.80%	6.00%
Janitorial supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	7.80%	7.40%	7.60%	7.30%
Electrical supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	4.60%	4.80%	4.60%	4.90%
Welding supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	4.10%	4.20%	4.10%	4.20%
Safety Supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	17.30%	16.40%	17.00%	16.10%
Other
Percentage of Sales by Product Line [Line Items]
Percentage of sales	3.30%	3.00%	3.10%	3.00%