|**Commitments and Contingencies (Tables)**              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|**Commitments and Contingencies Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Minimum commitments                                     | Minimum commitments as ofDecember 31, 2017consist of the following:(millions)Drilling, Equipment,and Purchase ObligationsTransportationand Gathering ObligationsOperatingLeaseObligationsCapitalLease and Other Obligations(1)Total2018$636$215$44$74$96920191672523345497202040247324236120211322332292972022818233212442023 and Thereafter321,3551561241,667Total$896$2,474$330$335$4,035(1)Annual lease payments, net to our interest, exclude regular maintenance and operational costs. SeeNote10.  Long-Term Debt.
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|(millions)                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2018                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2019                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2020                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2021                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2022                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|2023 and Thereafter                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Total                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
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|(1)                                                     | Annual lease payments, net to our interest, exclude regular maintenance and operational costs. SeeNote10.  Long-Term Debt.                                                                                                                                                                                                                                                                                                                                                                                              

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duration X- DefinitionTabular disclosure of the aggregate amount of payments due on known contractual obligations for the five years following the date of the latest balance sheet and the combined aggregate amount of maturities of known contractual obligations.
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