Business Segments - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue from External Customer [Line Items]
Segment revenues	$ 51,147	$ 51,069	$ 45,000
Operating Segments | Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	15,116	16,255	10,242
Operating Segments | Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	2,662	6,096	7,810
Operating Segments | Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	870	1,216	964
Operating Segments | Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	1,263	1,043	819
Operating Segments | Mineral resources | Land lease for oil exploration
Revenue from External Customer [Line Items]
Segment revenues	156	198	648
Operating Segments | Mineral resources | Water sales
Revenue from External Customer [Line Items]
Segment revenues	10,165	7,702	0
Operating Segments | Mineral resources | Other
Revenue from External Customer [Line Items]
Segment revenues	$ 0	$ 0	$ 1