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Inventories and natural gas in storage (Details) (USD $)
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Inventories and natural gas in storage [Abstract]      
Aggregates held for resale $ 88,632,000 $ 78,518,000 $ 80,868,000
Asphalt oil 47,084,000 32,335,000 26,851,000
Materials and supplies 75,551,000 61,611,000 64,988,000
Merchandise for resale 30,827,000 32,165,000 30,974,000
Natural gas in storage (current) 41,091,000 36,578,000 39,629,000
Other 32,582,000 32,998,000 26,259,000
Total 315,767,000 274,205,000 269,569,000
Natural gas in storage noncurrent $ 50,300,000 $ 50,300,000 $ 47,200,000