Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue from External Customer [Line Items]
Total expenses	$ 12,125	$ 12,717	$ 34,160	$ 32,060
Operating results	(2,155)	3,050	(3,856)	2,851
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	1,559	1,355	8,351	11,986
Total expenses	576	574	5,006	5,400
Operating results	$ 983	$ 781	$ 3,345	$ 6,586

Total costs of sales and operating expenses for the period.

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