Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Total revenues	$ 13,513	$ 9,657	$ 28,601	$ 29,265
Total expenses	14,340	12,125	33,974	34,160
Loss from operations before equity in earnings of unconsolidated joint ventures	(306)	(2,155)	(3,144)	(3,856)
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	1,322	1,559	9,276	8,351
Total expenses	648	576	5,240	5,006
Loss from operations before equity in earnings of unconsolidated joint ventures	$ 674	$ 983	$ 4,036	$ 3,345