Inventories and natural gas in storage (Details 2) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Abstract]
Aggregates held for resale	$ 87,715,000	$ 78,518,000
Materials and supplies	69,390,000	61,611,000
Natural gas in storage (current)	29,030,000	36,578,000
Asphalt oil	67,480,000	32,335,000
Merchandise for resale	31,172,000	32,165,000
Other	32,628,000	32,998,000
Inventories and natural gas in storage	317,415,000	274,205,000
Natural gas in storage noncurrent	$ 49,700,000	$ 50,300,000