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Summary of significant accounting policies - Computation of Depreciation of Property and Equipment (Detail) | 12 Months Ended
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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

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- Definition Describes an entity's methodology for allocating the cost of a property, plant or equipment, less salvage value (if any), over the estimated useful life of the asset in a systematic and rational manner. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section C -Paragraph 5 -Chapter 9 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -Subparagraph d -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
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