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Summary of Significant Accounting Policies (Concentration Risk) (Details) | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Sales and Rental Income |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 10.00% |  |
Accounts Receivable |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 10.00% |  |
Devon | Sales and Rental Income | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 21.00% | 18.00% | 15.00%
Devon | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 10.00% |  |
Oxy | Sales and Rental Income | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 10.00% |  |
Oxy | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 25.00% | 14.00% |
EOG | Sales and Rental Income | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  | 15.00% | 18.00%
Promatcon | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  | 20.00% |
Tepatguna | Accounts Receivable | Customer Concentration Risk |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  | 18.00% |

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