Inventories and natural gas in storage (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 130,544	$ 115,854	$ 123,457
Asphalt oil	42,591	36,498	79,422
Natural gas in storage (current)	19,689	21,023	11,310
Materials and supplies	20,765	16,997	22,594
Merchandise for resale	18,439	15,318	16,140
Other	33,821	34,861	37,316
Total	265,849	240,551	290,239
Natural gas in storage noncurrent	$ 49,100	$ 49,100	$ 49,300