Commitments and Contingencies (Details) $ in Millions	Dec. 31, 2017 USD ($)
Long-term Commitment [Line Items]
Operating Leases, Future Minimum Payments Due, Current Year	$ 18
Operating Leases, Future Minimum Payments, Due in Two Years	16
Operating Leases, Future Minimum Payments, Due in Three Years	16
Operating Leases, Future Minimum Payments, Due in Four Years	15
Operating Leases, Future Minimum Payments, Due in Five Years	15
Operating Leases, Future Minimum Payments, Due Thereafter	46
Operating Leases, Future Minimum Payments Due	126
Total Commitment, Due in Next Twelve Months	820
Total Commitment, Due in Second Year	756
Total Commitment, Due in Third Year	707
Total Commitment, Due in Fourth Year	594
Total Commitment, Due in Fifth Year	544
Total Commitment, Due Thereafter	2,361
Total Commitment	5,782
Transportation And Processing Commitments [Member]
Long-term Commitment [Line Items]
Purchase Obligation, Due in Next Twelve Months	604
Purchase Obligation, Due in Second Year	701
Purchase Obligation, Due in Third Year	670
Purchase Obligation, Due in Fourth Year	571
Purchase Obligation, Due in Fifth Year	529
Purchase Obligation, Due after Fifth Year	2,315
Purchase Obligation	5,390
Drilling And Field Services Commitments [Member]
Long-term Commitment [Line Items]
Purchase Obligation, Due in Next Twelve Months	198
Purchase Obligation, Due in Second Year	39
Purchase Obligation, Due in Third Year	21
Purchase Obligation, Due in Fourth Year	8
Purchase Obligation	$ 266