|**Commitments and Contingencies (Tables)**              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|------------------------------------------------------- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Commitments and Contingencies Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Minimum commitments                                     | Minimum commitments as ofDecember 31, 2016consist of the following:(millions)Drilling, Equipment,and Purchase ObligationsTransportationand Gathering ObligationsOperatingLeaseObligationsCapitalLease and Other Obligations(1)Total2017$255$250$30$77$6122018963124279529201952314305244820202727528523822021923728383122022 and Thereafter301,5661881631,947Total$469$2,954$346$461$4,230(1)Annual lease payments, net to our interest, exclude regular maintenance and operational costs. SeeNote10.  Long-Term Debt.
|                                                       
|                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(millions)                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2017                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2018                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2019                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2020                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2021                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|2022 and Thereafter                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Total                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|(1)                                                     | Annual lease payments, net to our interest, exclude regular maintenance and operational costs. SeeNote10.  Long-Term Debt.                                                                                                                                                                                                                                                                                                                                                                                             

X- ReferencesNo definition available.
+ Details**Name:**
us-gaap_CommitmentsAndContingenciesDisclosureAbstract**Namespace Prefix:**
us-gaap_**Data Type:**
xbrli:stringItemType**Balance Type:**
na**Period Type:**
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.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 303 -Paragraph a -Subparagraph 5
+ Details**Name:**
us-gaap_ContractualObligationFiscalYearMaturityScheduleTableTextBlock**Namespace Prefix:**
us-gaap_**Data Type:**
nonnum:textBlockItemType**Balance Type:**
na**Period Type:**
duration