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Inventories and natural gas in storage (Details) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2012
Inventories and natural gas in storage [Abstract]      
Aggregates held for resale $ 103,503,000 $ 87,715,000 $ 90,992,000
Asphalt oil 91,837,000 67,480,000 81,915,000
Materials and supplies 74,648,000 69,390,000 72,321,000
Merchandise for resale 27,330,000 31,172,000 30,417,000
Natural gas in storage (current) 14,287,000 29,030,000 26,216,000
Other 34,280,000 32,628,000 31,531,000
Total 345,885,000 317,415,000 333,392,000
Natural gas in storage noncurrent $ 48,600,000 $ 49,700,000 $ 50,300,000