Inventory (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Inventory [Line Items]
Percentage of LIFO Inventory	90.00%	90.00%
Excess of Replacement or Current Costs over Stated LIFO Value	$ 170,885	$ 147,091
Inventory, LIFO Reserve, Effect on Income, Net		131
Inventory Valuation Reserves	9,220	6,553
Inventory Valuation Reserve
Inventory [Line Items]
Reserves for Obsolete Inventories, Period Increase (Decrease)	$ 2,667	$ 1,641	$ 495