Property, Plant and Mine Development - Schedule of Property, Plant and Mine Development (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Property, Plant and Equipment
Interest Costs Capitalized	$ 90	$ 42
Property, plant and mine development	776,025		$ 770,585
Accumulated depletion, depreciation and amortization	(223,020)		(209,389)
Total property, plant and mine development, net	553,005		561,196
Mining Property and Mine Development
Property, Plant and Equipment
Property, plant and mine development	222,427		222,439
Asset Retirement Cost
Property, Plant and Equipment
Property, plant and mine development	9,887		9,889
Land
Property, Plant and Equipment
Property, plant and mine development	30,322		30,322
Land Improvements
Property, Plant and Equipment
Property, plant and mine development	37,972		37,791
Buildings
Property, Plant and Equipment
Property, plant and mine development	51,280		51,280
Machinery and Equipment
Property, Plant and Equipment
Property, plant and mine development	368,353		360,817
Furniture and Fixtures
Property, Plant and Equipment
Property, plant and mine development	1,917		1,917
Construction-in-Progress
Property, Plant and Equipment
Property, plant and mine development	$ 53,867		$ 56,130