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Summary of Significant Accounting Policies (Concentration Risk) (Details) - Customer Concentration Risk | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Devon | Sales and Rental Income |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 21.00% | 21.00% | 18.00%
Devon | Accounts Receivable |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  | 10.00% |
Oxy | Sales and Rental Income |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 19.00% | 10.00% |
Oxy | Accounts Receivable |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 15.00% | 25.00% |
EOG | Sales and Rental Income |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage |  |  | 15.00%
APR Energy LLC | Accounts Receivable |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 15.00% |  |
BP America | Accounts Receivable |  |  |
Concentration Risk [Line Items] |  |  |
Concentration risk, percentage | 14.00% |  |

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