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Inventories and natural gas in storage (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 106,623,000	$ 101,568,000	$ 104,784,000
Asphalt oil	33,551,000	38,099,000	43,078,000
Materials and supplies	71,515,000	69,808,000	71,370,000
Merchandise for resale	24,566,000	21,720,000	23,713,000
Natural gas in storage (current)	29,979,000	16,417,000	37,689,000
Other	36,471,000	34,779,000	33,937,000
Total	302,705,000	282,391,000	314,571,000
Natural gas in storage noncurrent	$ 47,400,000	$ 48,300,000	$ 48,600,000

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- Definition

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

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

+ References

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Carrying amount as of the balance sheet date of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, and deemed to be noncurrent asset.

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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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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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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Carrying amount, net of valuation reserves and allowances, as of the balance sheet date of inventories not otherwise specified in the taxonomy.

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