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Summary of Significant Accounting Policies (Tables)
12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]  
Computation of Depreciation of Property and Equipment Property and equipment; depreciation expense. We state property and equipment at acquisition cost, including capitalized interest on borrowings during the actual construction period of major capital projects. In 2016, 2017 and 2018 we capitalized $1.0 million, $2.2 million and $1.1 million, respectively, of interest costs. We compute depreciation of property and equipment for financial reporting purposes (including mining equipment) principally by the straight-line method over the estimated useful lives of the assets as follows:

 

Asset

  

Useful lives

Buildings and improvements

  

10 to 40 years

Machinery and equipment

  

3 to 20 years

Mine development costs

  

Units-of-production