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Inventories and natural gas in storage (Details) (USD $)
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Inventory Disclosure [Abstract]      
Aggregates held for resale $ 104,784,000 $ 87,715,000 $ 88,632,000
Asphalt oil 43,078,000 67,480,000 47,084,000
Materials and supplies 71,370,000 69,390,000 75,551,000
Merchandise for resale 23,713,000 31,172,000 30,827,000
Natural gas in storage (current) 37,689,000 29,030,000 41,091,000
Other 33,937,000 32,628,000 32,582,000
Total 314,571,000 317,415,000 315,767,000
Natural gas in storage noncurrent $ 48,600,000 $ 49,700,000 $ 50,300,000