Inventories and natural gas in storage (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Inventories [Abstract]
Aggregates held for resale	$ 82,936,000	$ 79,894,000	$ 83,535,000
Materials and supplies	65,363,000	57,324,000	62,070,000
Natural gas in storage (current)	11,993,000	34,557,000	14,269,000
Merchandise for resale	33,435,000	30,182,000	28,438,000
Asphalt oil	55,729,000	25,234,000	51,956,000
Other	27,871,000	25,706,000	19,895,000
Total	277,327,000	252,897,000	260,163,000
Natural gas in storage largely required to maintain pressure levels for normal operating purposes	$ 47,200,000	$ 48,000,000	$ 59,300,000