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Derivative Instruments Notional Amounts of Derivative Instruments (Details) - MMcf MMcf in Billions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Commodity Member [Member]
Derivative [Line Items]
Notional amount	1,143.5	1,067.2
Basis Swap [Member]
Derivative [Line Items]
Notional amount	766.9	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
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- Definition

Nominal volume used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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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:

X

- Details

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