Notional Amounts of Derivative Instruments (Details) - MMcf MMcf in Billions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Commodity Member [Member]
Derivative [Line Items]
Notional amount	1,096.1	1,067.2
Basis Swap [Member]
Derivative [Line Items]
Notional amount	781.8	688.1

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.

No definition available.

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Period Type:	duration