Summary of Significant Accounting Policies - Computation of Depreciation of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Buildings and improvements | Minimum
Property Plant And Equipment [Line Items]
Property and equipment useful life	10 years
Buildings and improvements | Maximum
Property Plant And Equipment [Line Items]
Property and equipment useful life	40 years
Machinery and equipment | Minimum
Property Plant And Equipment [Line Items]
Property and equipment useful life	3 years
Machinery and equipment | Maximum
Property Plant And Equipment [Line Items]
Property and equipment useful life	20 years
Mine development costs
Property Plant And Equipment [Line Items]
Property and equipment useful life	Units-of-production