Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 119,078	$ 115,854	$ 115,736
Asphalt oil	23,480	36,498	33,581
Natural gas in storage (current)	35,625	21,023	28,222
Materials and supplies	18,584	16,997	19,404
Merchandise for resale	15,672	15,318	15,563
Other	33,351	34,861	33,481
Total	245,790	240,551	245,987
Natural gas in storage noncurrent	$ 49,100	$ 49,100	$ 49,300