|**Commitments and Contingencies (Tables)**              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|**Commitments and Contingencies Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Minimum commitments                                     | Minimum commitments as of December 31, 2018consist of the following:(millions)Purchase and Service ObligationsMarcellus Shale Firm Transportation and Other Obligations(1)Gathering, Transportation & Processing ObligationsOperatingLeaseObligations(2)CapitalLease Obligations(2)Total2019$197$123$151$91$52$61420202912212974464002021131211035931327202261186762222752023211136650202702024 and Thereafter59342851761041,504Total$271$1,531$801$512$275$3,390(1)Amount includes exit cost obligations resulting from a permanent capacity assignment. SeeNote10.  Marcellus Shale Firm Transportation Commitments.(2)Annual lease payments, net to our interest, exclude regular maintenance and operational costs. SeeNote9.  Long-Term Debt.
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|(millions)                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2019                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2020                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2021                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2022                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2023                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|2024 and Thereafter                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|(1)                                                     | Amount includes exit cost obligations resulting from a permanent capacity assignment. SeeNote10.  Marcellus Shale Firm Transportation Commitments.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

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