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Summary of Significant Accounting Policies Gas Imbalances (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |
Gas imbalance receivable | $ 42 | $ 108
Gas imbalance payable | $ 47 | $ 45

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- Definition The value of gas balancing asset or liability as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=82851474&loc=d3e512571-122862 + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Carrying value as of the balance sheet date of obligations incurred and payable to an underproduced working interest owner when an overproduced working interest owner has sold gas in excess of the party's ownership percentage. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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