Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Nature of Operations
Ownership percentage of subsidiaries	100.00%
Impairment of Long-Lived Assets Held-for-use	$ 0	$ 0
Buildings | Minimum
Property, plant and equipment and long-lived asset impairment
Useful life	30 years
Buildings | Maximum
Property, plant and equipment and long-lived asset impairment
Useful life	40 years
Mining equipment and machinery | Minimum
Property, plant and equipment and long-lived asset impairment
Useful life	3 years
Mining equipment and machinery | Maximum
Property, plant and equipment and long-lived asset impairment
Useful life	8 years
Other furniture and equipment | Minimum
Property, plant and equipment and long-lived asset impairment
Useful life	3 years
Other furniture and equipment | Maximum
Property, plant and equipment and long-lived asset impairment
Useful life	5 years