Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue from External Customer [Line Items]
Total expenses	$ 12,076	$ 11,694
Operating results	(1,527)	(657)
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	6,178	6,132
Total expenses	3,878	3,832
Operating results	$ 2,300	$ 2,300