Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Total revenues	$ 49,523	$ 45,619	$ 35,255
Total expenses	50,954	45,183	42,773
Operating results	(2,016)	1,721	(7,331)
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	9,791	14,395	5,983
Total expenses	5,818	6,223	2,964
Operating results	$ 3,973	$ 8,172	$ 3,019