Supplemental Information On Oil And Gas Operations (Proved Natural Gas Reserves) (Details) Mcf in Millions	12 Months Ended
	Dec. 31, 2016 Mcf	Dec. 31, 2015 Mcf	Dec. 31, 2014 Mcf	Dec. 31, 2013 Mcf
Reserve Quantities [Line Items]
Conversion rate of gas reserves from barrels of oil to Boe	6
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance	5,821	7,687	9,308
Proved developed and undeveloped reserves, revisions due to prices	(103)	(1,421)	236
Proved developed and undeveloped reserves, revisions other than price	638	(9)	(295)
Proved developed and undeveloped reserves, extensions and discoveries	280	171	343
Proved developed and undeveloped reserves, purchase of reserves	33	17	457
Proved developed and undeveloped reserves, production	(517)	(587)	(701)
Proved developed and undeveloped reserves, sale of reserves	(521)	(37)	(1,661)
Proved developed and undeveloped reserves, ending balance	5,631	5,821	7,687
Proved developed reserves	5,377	5,707	6,984	8,459
Proved developed producing reserves	5,259	5,559	6,780	8,105
Proved undeveloped reserves	254	114	703	849
Natural Gas [Member] | United States [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance	5,808	7,651	8,550
Proved developed and undeveloped reserves, revisions due to prices	(103)	(1,412)	191
Proved developed and undeveloped reserves, revisions other than price	628	(3)	(299)
Proved developed and undeveloped reserves, extensions and discoveries	280	171	335
Proved developed and undeveloped reserves, purchase of reserves	33	17	457
Proved developed and undeveloped reserves, production	(510)	(579)	(660)
Proved developed and undeveloped reserves, sale of reserves	(521)	(37)	(923)
Proved developed and undeveloped reserves, ending balance	5,615	5,808	7,651
Proved developed reserves	5,361	5,694	6,948	7,707
Proved developed producing reserves	5,243	5,546	6,746	7,425
Proved undeveloped reserves	254	114	703	843
Natural Gas [Member] | Canada [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance	13	36	758
Proved developed and undeveloped reserves, revisions due to prices		(9)	45
Proved developed and undeveloped reserves, revisions other than price	10	(6)	4
Proved developed and undeveloped reserves, extensions and discoveries			8
Proved developed and undeveloped reserves, production	(7)	(8)	(41)
Proved developed and undeveloped reserves, sale of reserves			(738)
Proved developed and undeveloped reserves, ending balance	16	13	36
Proved developed reserves	16	13	36	752
Proved developed producing reserves	16	13	34	680
Proved undeveloped reserves				6