Inventories and natural gas in storage (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 112,129,000	$ 101,568,000	$ 103,503,000
Asphalt oil	76,525,000	38,099,000	91,837,000
Materials and supplies	70,938,000	69,808,000	74,648,000
Merchandise for resale	25,507,000	21,720,000	27,330,000
Natural gas in storage (current)	10,903,000	16,417,000	14,287,000
Other	35,420,000	34,779,000	34,280,000
Total	331,422,000	282,391,000	345,885,000
Natural gas in storage noncurrent	$ 47,400,000	$ 48,300,000	$ 48,600,000