Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue from External Customer [Line Items]
Total expenses	$ 7,558	$ 10,341	$ 19,634	$ 22,035
Operating results	(1,311)	(1,044)	(2,838)	(1,701)
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	1,776	660	7,954	6,792
Total expenses	714	598	4,592	4,430
Operating results	$ 1,062	$ 62	$ 3,362	$ 2,362