Commitments and Contingencies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) plaintiff item
Commitments And Contingencies [Line Items]
Obligation under transportation agreements	$ 9,171,000,000
Guarantee obligations relative to the firms transportation agreements and gathering project and services	$ 832,000,000
Lease expiration date	Dec. 31, 2027
Operating leases, future minimum payments, 2018	$ 65,000,000
Operating leases, future minimum payments, 2019	58,000,000
Operating leases, future minimum payments, 2020	47,000,000
Operating leases, future minimum payments, 2021	28,000,000
Operating leases, future minimum payments, 2022	4,000,000
Operating leases, future minimum payments, thereafter	11,000,000
Indemnification liability	$ 0
Arkansas Royalty Litigation [Member] | Arkansas State Court [Member]
Commitments And Contingencies [Line Items]
Number of actions | item	2
Number of state cases set for trial | item	1
Number of cases | item	3
Number of plaintiffs | plaintiff	248
Number of cases removed to federal court | item	2
Pending Regulatory Approval and/or Construction [Member]
Commitments And Contingencies [Line Items]
Obligation under transportation agreements	$ 2,936,000,000
Exploration and Production [Member] | Pressure Pumping Equipment [Member]
Commitments And Contingencies [Line Items]
Lease expiration date	Dec. 31, 2021
Aggregate annual lease payment	$ 7,000,000
Exploration and Production [Member] | Drilling Rigs [Member]
Commitments And Contingencies [Line Items]
Number of leases | item	7
Lease expiration date	Dec. 31, 2021
Aggregate annual lease payment	$ 13,000,000
Midstream Services and E&P [Member]
Commitments And Contingencies [Line Items]
Operating leases, future minimum payments, 2018	12,000,000
Operating leases, future minimum payments, 2019	$ 3,000,000