Revenue - Percentages of Sales by Product Line (Details) - Product Line - Sales	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Percentage of Sales by Product Line [Line Items]
Percentage of sales	100.00%	100.00%	100.00%
Fasteners
Percentage of Sales by Product Line [Line Items]
Percentage of sales	34.90%	35.60%	36.60%
Tools
Percentage of Sales by Product Line [Line Items]
Percentage of sales	10.00%	10.10%	9.90%
Cutting tools
Percentage of Sales by Product Line [Line Items]
Percentage of sales	5.70%	5.80%	5.70%
Hydraulics and pneumatics
Percentage of Sales by Product Line [Line Items]
Percentage of sales	6.80%	6.80%	6.90%
Material handling
Percentage of Sales by Product Line [Line Items]
Percentage of sales	5.80%	5.90%	6.00%
Janitorial supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	7.60%	7.30%	7.30%
Electrical supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	4.70%	4.90%	4.80%
Welding supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	4.10%	4.20%	4.20%
Safety supplies
Percentage of Sales by Product Line [Line Items]
Percentage of sales	17.20%	16.30%	16.00%
Other
Percentage of Sales by Product Line [Line Items]
Percentage of sales	3.20%	3.10%	2.60%