Summary of significant accounting policies - Computation of Depreciation of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2012
Mine development costs [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	Units-of-production
Minimum [Member] | Buildings and improvements [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	10 years
Minimum [Member] | Machinery and equipment [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Minimum [Member] | Landfill disposal costs [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	15 years
Maximum [Member] | Buildings and improvements [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	40 years
Maximum [Member] | Machinery and equipment [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	20 years
Maximum [Member] | Landfill disposal costs [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Property and equipment useful life	39 years