Inventories (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Inventory, Net [Abstract]
Raw materials	$ 48,735	$ 45,408	$ 45,209
Work in progress	11,130	10,879	10,321
Finished goods	184,871	167,416	153,040
Subtotal	244,736	223,703	208,570
LIFO and valuation adjustments	(25,603)	(22,112)	(24,012)
Inventories, net	$ 219,133	$ 201,591	$ 184,558

Amount of inventory reserves for last-in first-out (LIFO) and other inventory valuation methods.

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Amount before valuation and LIFO reserves of completed merchandise or goods expected to be sold within one year or operating cycle, if longer.

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Gross amount, as of the balance sheet date, of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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Amount before valuation and LIFO reserves of raw materials expected to be sold, or consumed within one year or operating cycle, if longer.

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Amount before valuation and LIFO reserves of merchandise or goods in the production process expected to be completed within one year or operating cycle, if longer.

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