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Summary of Significant Accounting Policies (Concentration Risk) (Details) - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Oxy | Sales and Rental Income
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%	19.00%	10.00%
Oxy | Accounts Receivable
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	15.00%
Devon | Sales and Rental Income
Concentration Risk [Line Items]
Concentration risk, percentage	15.00%	21.00%	21.00%
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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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

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