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Accounting Policies - Cash and Cash Equivalents and Temporary Investments (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
U.S. dollar    
Cash and Cash Equivalents and Temporary Investments    
Average yield for cash equivalents and temporary investments 0.87% 0.36%
Mexican peso    
Cash and Cash Equivalents and Temporary Investments    
Average yield for cash equivalents and temporary investments 6.72% 4.06%