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Inventories and natural gas in storage (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Inventories and natural gas in storage [Abstract]
Aggregates held for resale	$ 98,120,000	$ 87,715,000	$ 85,958,000
Asphalt oil	94,332,000	67,480,000	82,949,000
Materials and supplies	75,868,000	69,390,000	68,369,000
Merchandise for resale	24,342,000	31,172,000	28,459,000
Natural gas in storage (current)	12,811,000	29,030,000	15,475,000
Other	29,399,000	32,628,000	32,131,000
Total	334,872,000	317,415,000	313,341,000
Natural gas in storage noncurrent	$ 49,600,000	$ 49,700,000	$ 50,300,000

X

- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

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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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.5-02.6(a))

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-Publisher SEC

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-Number 210

-Section 02

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-Subparagraph a

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- Definition

Amount before last-in first-out (LIFO) and valuation reserves of merchandise or goods held by the entity that are readily available for sale.

+ References

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

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-Name Regulation S-X (SX)

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- Definition

Carrying amount (lower of cost or market) as of the balance sheet date of inventories less all valuation and other allowances. Excludes noncurrent inventory balances (expected to remain on hand past one year or one operating cycle, if longer).

+ References

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- Definition

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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