Equity Investments - Additional Information (Details) - Western Gas Partners, LP [Member] - Fort Union [Member] $ in Millions	Sep. 30, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity investment difference between carrying and underlying value	$ 3.1
Fair Value, Measurements, Nonrecurring [Member] \| Level 3 Inputs [Member] \| Income Approach Valuation Technique [Member]
Schedule of Equity Method Investments [Line Items]
Equity investment fair value	$ 8.5

Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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- Definition

Fair value portion of investments accounted under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66022186&loc=d3e32014-111567

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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