Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Net operating income (loss)	$ (2,013)	$ (2,235)	$ (199)	$ (5,708)
Mineral resources
Revenue from External Customer [Line Items]
Commercial/industrial revenues	1,500	1,519	10,631	3,520
Net operating income (loss)	905	990	5,805	1,667
Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Commercial/industrial revenues	623	429	1,161	805
Mineral resources | Cement
Revenue from External Customer [Line Items]
Commercial/industrial revenues	477	519	828	797
Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Commercial/industrial revenues	344	248	546	428
Mineral resources | Exploration leases
Revenue from External Customer [Line Items]
Commercial/industrial revenues		0
Mineral resources | Water Sales
Revenue from External Customer [Line Items]
Commercial/industrial revenues	0	146	7,992	1,254
Mineral resources | Reimbursables and other
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ 56	$ 177	$ 104	$ 236