Reporting Segments and Related Information - Revenue Components of Mineral Resources Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Revenue from External Customer [Line Items]
Total expenses	$ 11,694	$ 11,924
Operating results	(657)	1,814
Mineral resources
Revenue from External Customer [Line Items]
Total revenues	6,132	9,131
Total expenses	3,832	4,231
Operating results	$ 2,300	$ 4,900

Total costs of sales and operating expenses for the period.

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