Inventories and natural gas in storage (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Inventories and natural gas in storage [Abstract]
Aggregates held for resale	$ 90,992,000	$ 78,518,000	$ 82,936,000
Asphalt oil	81,915,000	32,335,000	55,729,000
Materials and supplies	72,321,000	61,611,000	65,363,000
Merchandise for resale	30,417,000	32,165,000	33,435,000
Natural gas in storage (current)	26,216,000	36,578,000	11,993,000
Other	31,531,000	32,998,000	27,871,000
Total	333,392,000	274,205,000	277,327,000
Natural gas in storage noncurrent	$ 50,300,000	$ 50,300,000	$ 47,200,000