Inventories and natural gas in storage (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Inventories [Abstract]
Aggregates held for resale	$ 80,868,000	$ 79,894,000	$ 82,622,000
Materials and supplies	64,988,000	57,324,000	62,273,000
Natural gas in storage (current)	39,629,000	34,557,000	40,133,000
Merchandise for resale	30,974,000	30,182,000	29,566,000
Asphalt oil	26,851,000	25,234,000	24,341,000
Other	26,259,000	25,706,000	22,745,000
Total	269,569,000	252,897,000	261,680,000
Natural gas in storage largely required to maintain pressure levels for normal operating purposes	$ 47,200,000	$ 48,000,000	$ 59,300,000