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Notional Amounts of Derivative Instruments (Details) - MMcf MMcf in Billions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Commodity Member [Member] |  |
Derivative [Line Items] |  |
Notional amount | 1,064.1 | 1,067.2
Basis Swap [Member] |  |
Derivative [Line Items] |  |
Notional amount | 775.5 | 688.1

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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. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Nominal volume used to calculate payments on a derivative instrument. + References No definition available. + Details
Name: | us-gaap_DerivativeNonmonetaryNotionalAmountVolume
Namespace Prefix: | us-gaap_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_BasisSwapMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |