Business Segments - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Segment revenues	$ 45,577	$ 51,147	$ 51,069
Operating Segments | Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	14,153	15,116	16,255
Operating Segments | Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	1,549	2,661	6,096
Operating Segments | Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	1,164	870	1,216
Operating Segments | Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	1,299	1,263	1,043
Operating Segments | Mineral resources | Land lease for oil exploration
Revenue from External Customer [Line Items]
Segment revenues	176	157	198
Operating Segments | Mineral resources | Water sales
Revenue from External Customer [Line Items]
Segment revenues	9,601	10,165	7,702
Operating Segments | Mineral resources | Reimbursable costs
Revenue from External Customer [Line Items]
Segment revenues	$ 364	$ 0	$ 0