Inventories and natural gas in storage (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Inventories and natural gas in storage [Abstract]
Aggregates held for resale	$ 85,958,000	$ 78,518,000	$ 82,086,000
Asphalt oil	82,949,000	32,335,000	51,506,000
Materials and supplies	68,369,000	61,611,000	61,788,000
Merchandise for resale	28,459,000	32,165,000	31,830,000
Natural gas in storage (current)	15,475,000	36,578,000	11,953,000
Other	32,131,000	32,998,000	23,533,000
Total	313,341,000	274,205,000	262,696,000
Natural gas in storage noncurrent	$ 50,300,000	$ 50,300,000	$ 47,200,000