Property, Plant, Equipment and Mineral Interests (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Movement in Property, Plant and Equipment:
Begining balance	$ 171,784	$ 164,425
Additions	2,499	7,572
Transfers		0
Disposals	(50)	(213)
Ending balance	174,233	171,784
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	35,966	22,577
Additions	3,498	13,536
Disposals		(147)
Ending balance	39,464	35,966
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	134,769	135,818
Land [Member]
Movement in Property, Plant and Equipment:
Begining balance	4,137	3,969
Additions	132	173
Transfers		(5)
Disposals	0	0
Ending balance	4,269	4,137
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	0	0
Additions	0	0
Disposals		0
Ending balance	0	0
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	4,269	4,137
Mineral property interest and claims [Member]
Movement in Property, Plant and Equipment:
Begining balance	5,835	5,280
Additions	0	5
Transfers		550
Disposals	0	0
Ending balance	5,835	5,835
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	606	328
Additions	55	278
Disposals		0
Ending balance	661	606
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	5,174	5,229
Mine development [Member]
Movement in Property, Plant and Equipment:
Begining balance	51,976	50,773
Additions	416	492
Transfers		711
Disposals	0	0
Ending balance	52,392	51,976
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	6,097	3,415
Additions	772	2,682
Disposals		0
Ending balance	6,869	6,097
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	45,523	45,879
Machinery and equipment [Member]
Movement in Property, Plant and Equipment:
Begining balance	75,228	70,066
Additions	89	0
Transfers		5,375
Disposals	0	(213)
Ending balance	75,317	75,228
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	20,995	13,353
Additions	1,981	7,789
Disposals		(147)
Ending balance	22,976	20,995
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	52,341	54,233
Buildings and infrastructure [Member]
Movement in Property, Plant and Equipment:
Begining balance	28,445	28,397
Additions	0	0
Transfers		48
Disposals	(50)	0
Ending balance	28,395	28,445
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	7,367	5,010
Additions	582	2,357
Disposals		0
Ending balance	7,949	7,367
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	20,446	21,078
Construction in progress [Member]
Movement in Property, Plant and Equipment:
Begining balance	277	54
Additions	1,862	6,902
Transfers		(6,679)
Disposals	0	0
Ending balance	2,139	277
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	0	0
Additions	0	0
Disposals		0
Ending balance	0	0
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	2,139	277
Interest capitalized [Member]
Movement in Property, Plant and Equipment:
Begining balance	5,886	5,886
Additions	0	0
Transfers		0
Disposals	0	0
Ending balance	5,886	5,886
Movement in Accumulated Depreciation, Depletion and Amortization, Property, Plant and Equipment:
Begining balance	901	471
Additions	108	430
Disposals		0
Ending balance	1,009	901
Property, Plant and Equipment, Net, by Type:
Property, Plant and Equipment, Net	$ 4,877	$ 4,985