Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Segment revenues	$ 35,657	$ 45,577	$ 51,147
Operating Segments
Revenue from External Customer [Line Items]
Segment revenues	35,657	45,577	51,147
Operating Segments | Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	5,983	14,153	15,116
Operating Segments | Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	1,659	1,549	2,661
Operating Segments | Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	1,072	1,164	870
Operating Segments | Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	1,614	1,299	1,263
Operating Segments | Mineral resources | Exploration leases
Revenue from External Customer [Line Items]
Segment revenues	102	176	157
Operating Segments | Mineral resources | Water sales
Revenue from External Customer [Line Items]
Segment revenues	1,254	9,601	10,165
Operating Segments | Mineral resources | Reimbursable
Revenue from External Customer [Line Items]
Segment revenues	$ 282	$ 364	$ 0