Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2014
Building and improvements | Minimum
Property Plant And Equipment Estimated Useful Life [Line Items]
Property plant and equipment useful life	10 years
Building and improvements | Maximum
Property Plant And Equipment Estimated Useful Life [Line Items]
Property plant and equipment useful life	40 years
Machinery and equipment | Minimum
Property Plant And Equipment Estimated Useful Life [Line Items]
Property plant and equipment useful life	3 years
Machinery and equipment | Maximum
Property Plant And Equipment Estimated Useful Life [Line Items]
Property plant and equipment useful life	20 years
Mine development costs
Property Plant And Equipment Estimated Useful Life [Line Items]
Mine development costs	units-of-production