Supplemental Oil and Gas Disclosures (Summary of Changes in Reserves - United States) (Details) ft³ in Billions	12 Months Ended
	Dec. 31, 2017 MMBbls ft³	Dec. 31, 2016 MMBbls ft³	Dec. 31, 2015 MMBbls ft³
Reserve Quantities [Line Items]
Proved reserves, beginning of year | ft³	5,253	6,215	10,747
Revisions of previous estimates | ft³	2,332	(354)	(4,083)
Extensions, discoveries and other additions | ft³	8,087	282	592
Production | ft³	(897)	(875)	(976)
Acquisition of reserves in place | ft³			115
Disposition of reserves in place | ft³		(15)	(180)
Proved reserves, end of year | ft³	14,775	5,253	6,215
Proved developed reserves:
Proved developed reserves | ft³	7,920	5,176	5,772
Proved undeveloped reserves:
Proved undeveloped reserves | ft³	6,855	77	443
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year | ft³	4,866	5,917	9,809
Revisions of previous estimates | ft³	1,898	(446)	(3,458)
Extensions, discoveries and other additions | ft³	5,159	198	546
Production | ft³	(797)	(788)	(899)
Acquisition of reserves in place | ft³			97
Disposition of reserves in place | ft³		(15)	(178)
Proved reserves, end of year | ft³	11,126	4,866	5,917
Proved developed reserves:
Proved developed reserves | ft³	6,979	4,789	5,474
Proved undeveloped reserves:
Proved undeveloped reserves | ft³	4,147	77	443
Oil [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year | MMBbls	10,523	8,753	37,615
Revisions of previous estimates | MMBbls	1,668	1,564	(28,394)
Extensions, discoveries and other additions | MMBbls	55,772	2,417	1,367
Production | MMBbls	(2,327)	(2,192)	(2,265)
Acquisition of reserves in place | MMBbls			525
Disposition of reserves in place | MMBbls		(19)	(95)
Proved reserves, end of year | MMBbls	65,636	10,523	8,753
Proved developed reserves:
Proved developed reserves | MMBbls	14,513	10,523	8,753
Proved undeveloped reserves:
Proved undeveloped reserves | MMBbls	51,123
NGL [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year | MMBbls	53,931	40,947	118,699
Revisions of previous estimates | MMBbls	70,549	13,794	(75,664)
Extensions, discoveries and other additions | MMBbls	432,220	11,576	6,274
Production | MMBbls	(14,245)	(12,372)	(10,702)
Acquisition of reserves in place | MMBbls			2,340
Disposition of reserves in place | MMBbls		(14)
Proved reserves, end of year | MMBbls	542,455	53,931	40,947
Proved developed reserves:
Proved developed reserves | MMBbls	142,213	53,931	40,947
Proved undeveloped reserves:
Proved undeveloped reserves | MMBbls	400,242