Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 133,477	$ 115,268	$ 116,399
Asphalt oil	40,781	30,360	26,682
Natural gas in storage (current)	29,084	20,950	29,974
Materials and supplies	23,563	18,650	20,778
Merchandise for resale	15,954	14,905	15,346
Other	27,434	26,450	23,376
Total	270,293	226,583	232,555
Natural gas in storage noncurrent	$ 47,800	$ 49,300	$ 49,500

Aggregated amount of aggregates held for resale. This amount is net of valuation reserves and adjustments.

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- Definition

Aggregated amount of asphalt oil inventory. This amount is net of valuation reserves and adjustments.

+ References

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- Definition

Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

+ References

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- Definition

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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Carrying amount as of the balance sheet date of natural gas stored underground in depleted gas reservoirs, aquifers, or salt caverns to meet future seasonal and peak load demands and also as insurance against unforeseen supply disruptions, and deemed to be a noncurrent asset because it is not expected to be used within 12 months or in the normal operating cycle.

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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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Gross amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

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

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