Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Segment revenues	$ 45,619	$ 35,255	$ 45,979
Mineral resources
Revenue from External Customer [Line Items]
Segment revenues	14,395	5,983	14,153
Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Segment revenues	2,278	1,659	1,549
Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Segment revenues	1,143	1,072	1,164
Mineral resources | Cement
Revenue from External Customer [Line Items]
Segment revenues	1,695	1,614	1,299
Mineral resources | Exploration leases
Revenue from External Customer [Line Items]
Segment revenues	102	102	176
Mineral resources | Water sales
Revenue from External Customer [Line Items]
Segment revenues	9,142	1,254	9,601
Mineral resources | Reimbursable
Revenue from External Customer [Line Items]
Segment revenues	$ 35	$ 282	$ 364