Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Net operating income (loss)	$ 3,050	$ (1,414)	$ 2,851	$ (7,122)
Mineral resources
Revenue from External Customer [Line Items]
Commercial/industrial revenues	1,355	1,142	11,986	4,662
Net operating income (loss)	781	614	6,586	2,281
Mineral resources | Oil and gas
Revenue from External Customer [Line Items]
Commercial/industrial revenues	584	356	1,745	1,161
Mineral resources | Cement
Revenue from External Customer [Line Items]
Commercial/industrial revenues	481	398	1,309	1,195
Mineral resources | Rock aggregate
Revenue from External Customer [Line Items]
Commercial/industrial revenues	301	262	847	690
Mineral resources | Exploration leases
Revenue from External Customer [Line Items]
Commercial/industrial revenues	26	0	77	76
Mineral resources | Water Sales
Revenue from External Customer [Line Items]
Commercial/industrial revenues	0	0	7,992	1,254
Mineral resources | Reimbursables and other
Revenue from External Customer [Line Items]
Commercial/industrial revenues	$ (37)	$ 126	$ 16	$ 286