Supplemental Information On Oil And Gas Operations (Proved Developed and Undeveloped Reserves) (Details) MBbls in Thousands, Mcf in Millions		12 Months Ended
		Dec. 31, 2017 MMBoe MBbls Mcf	Dec. 31, 2016 MMBoe MBbls Mcf	Dec. 31, 2015 MMBoe MBbls Mcf	Dec. 31, 2014 MMBoe MBbls Mcf
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | MMBoe	[1]	2,058	2,182	2,754
Proved developed and undeveloped reserves, revisions due to prices | MMBoe	[1]	73	(27)	(302)
Proved developed and undeveloped reserves, revisions other than price | MMBoe	[1]	(12)	137	(142)
Proved developed and undeveloped reserves, extensions and discoveries | MMBoe	[1]	237	126	118
Proved developed and undeveloped reserves, purchase of reserves | MMBoe	[1]		20	9
Proved developed and undeveloped reserves, production | MMBoe	[1]	(198)	(223)	(248)
Proved developed and undeveloped reserves, sale of reserves | MMBoe	[1]	(6)	(157)	(7)
Proved developed and undeveloped reserves, ending balance | MMBoe	[1]	2,152	2,058	2,182
Proved developed reserves | MMBoe	[1]	1,742	1,649	1,806	2,065
Proved developed producing reserves | MMBoe	[1]	1,693	1,593	1,749	1,977
Proved undeveloped reserves | MMBoe	[1]	410	409	376	689
Conversion rate of gas reserves from barrels of oil to Boe		6
United States [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | MMBoe	[1]	1,554	1,638	2,205
Proved developed and undeveloped reserves, revisions due to prices | MMBoe	[1]	111	(48)	(408)
Proved developed and undeveloped reserves, revisions other than price | MMBoe	[1]	(5)	151	(59)
Proved developed and undeveloped reserves, extensions and discoveries | MMBoe	[1]	221	124	104
Proved developed and undeveloped reserves, purchase of reserves | MMBoe	[1]		20	9
Proved developed and undeveloped reserves, production | MMBoe	[1]	(150)	(174)	(206)
Proved developed and undeveloped reserves, sale of reserves | MMBoe	[1]	(6)	(157)	(7)
Proved developed and undeveloped reserves, ending balance | MMBoe	[1]	1,725	1,554	1,638
Proved developed reserves | MMBoe	[1]	1,524	1,439	1,563	1,900
Proved developed producing reserves | MMBoe	[1]	1,481	1,386	1,509	1,815
Proved undeveloped reserves | MMBoe	[1]	201	115	75	305
Canada [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | MMBoe	[1]	504	544	549
Proved developed and undeveloped reserves, revisions due to prices | MMBoe	[1]	(38)	21	106
Proved developed and undeveloped reserves, revisions other than price | MMBoe	[1]	(7)	(14)	(83)
Proved developed and undeveloped reserves, extensions and discoveries | MMBoe	[1]	16	2	14
Proved developed and undeveloped reserves, production | MMBoe	[1]	(48)	(49)	(42)
Proved developed and undeveloped reserves, ending balance | MMBoe	[1]	427	504	544
Proved developed reserves | MMBoe	[1]	218	210	243	165
Proved developed producing reserves | MMBoe	[1]	212	207	240	162
Proved undeveloped reserves | MMBoe	[1]	209	294	301	384
Oil [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		211	264	374
Proved developed and undeveloped reserves, revisions due to prices		11	(20)	(49)
Proved developed and undeveloped reserves, revisions other than price		8	1	(50)
Proved developed and undeveloped reserves, extensions and discoveries		94	38	54
Proved developed and undeveloped reserves, purchase of reserves			8	5
Proved developed and undeveloped reserves, production		(49)	(55)	(70)
Proved developed and undeveloped reserves, sale of reserves		(3)	(25)
Proved developed and undeveloped reserves, ending balance		272	211	264
Proved developed reserves		193	177	225	278
Proved developed producing reserves		177	156	211	243
Proved undeveloped reserves		79	34	39	96
Oil [Member] | United States [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		194	242	351
Proved developed and undeveloped reserves, revisions due to prices		12	(18)	(53)
Proved developed and undeveloped reserves, revisions other than price		6	(2)	(52)
Proved developed and undeveloped reserves, extensions and discoveries		90	36	51
Proved developed and undeveloped reserves, purchase of reserves			8	5
Proved developed and undeveloped reserves, production		(42)	(47)	(60)
Proved developed and undeveloped reserves, sale of reserves		(3)	(25)
Proved developed and undeveloped reserves, ending balance		257	194	242
Proved developed reserves		178	160	203	255
Proved developed producing reserves		165	143	192	224
Proved undeveloped reserves		79	34	39	96
Oil [Member] | Canada [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		17	22	23
Proved developed and undeveloped reserves, revisions due to prices		(1)	(2)	4
Proved developed and undeveloped reserves, revisions other than price		2	3	2
Proved developed and undeveloped reserves, extensions and discoveries		4	2	3
Proved developed and undeveloped reserves, production		(7)	(8)	(10)
Proved developed and undeveloped reserves, ending balance		15	17	22
Proved developed reserves		15	17	22	23
Proved developed producing reserves		12	13	19	19
Bitumen [Member] | Canada [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		484	520	521
Proved developed and undeveloped reserves, revisions due to prices		(37)	23	103
Proved developed and undeveloped reserves, revisions other than price		(10)	(19)	(84)
Proved developed and undeveloped reserves, extensions and discoveries		12		11
Proved developed and undeveloped reserves, production		(40)	(40)	(31)
Proved developed and undeveloped reserves, ending balance		409	484	520
Proved developed reserves		200	190	219	137
Proved developed producing reserves		197	190	219	137
Proved undeveloped reserves		209	294	301	384
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | Mcf		5,631	5,821	7,687
Proved developed and undeveloped reserves, revisions due to prices | Mcf		399	(103)	(1,421)
Proved developed and undeveloped reserves, revisions other than price | Mcf		2	638	(9)
Proved developed and undeveloped reserves, extensions and discoveries | Mcf		403	280	171
Proved developed and undeveloped reserves, purchase of reserves | Mcf			33	17
Proved developed and undeveloped reserves, production | Mcf		(439)	(517)	(587)
Proved developed and undeveloped reserves, sale of reserves | Mcf		(9)	(521)	(37)
Proved developed and undeveloped reserves, ending balance | Mcf		5,987	5,631	5,821
Proved developed reserves | Mcf		5,632	5,377	5,707	6,984
Proved developed producing reserves | Mcf		5,525	5,259	5,559	6,780
Proved undeveloped reserves | Mcf		355	254	114	703
Natural Gas [Member] | United States [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | Mcf		5,615	5,808	7,651
Proved developed and undeveloped reserves, revisions due to prices | Mcf		398	(103)	(1,412)
Proved developed and undeveloped reserves, revisions other than price | Mcf			628	(3)
Proved developed and undeveloped reserves, extensions and discoveries | Mcf		403	280	171
Proved developed and undeveloped reserves, purchase of reserves | Mcf			33	17
Proved developed and undeveloped reserves, production | Mcf		(433)	(510)	(579)
Proved developed and undeveloped reserves, sale of reserves | Mcf		(9)	(521)	(37)
Proved developed and undeveloped reserves, ending balance | Mcf		5,974	5,615	5,808
Proved developed reserves | Mcf		5,619	5,361	5,694	6,948
Proved developed producing reserves | Mcf		5,512	5,243	5,546	6,746
Proved undeveloped reserves | Mcf		355	254	114	703
Natural Gas [Member] | Canada [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | Mcf		16	13	36
Proved developed and undeveloped reserves, revisions due to prices | Mcf		1		(9)
Proved developed and undeveloped reserves, revisions other than price | Mcf		2	10	(6)
Proved developed and undeveloped reserves, production | Mcf		(6)	(7)	(8)
Proved developed and undeveloped reserves, ending balance | Mcf		13	16	13
Proved developed reserves | Mcf		13	16	13	36
Proved developed producing reserves | Mcf		13	16	13	34
Natural Gas Liquids [Member] | United States [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		425	428	578
Proved developed and undeveloped reserves, revisions due to prices		32	(13)	(119)
Proved developed and undeveloped reserves, revisions other than price		(10)	48	(6)
Proved developed and undeveloped reserves, extensions and discoveries		63	42	24
Proved developed and undeveloped reserves, purchase of reserves			7	1
Proved developed and undeveloped reserves, production		(36)	(42)	(50)
Proved developed and undeveloped reserves, sale of reserves		(1)	(45)
Proved developed and undeveloped reserves, ending balance		473	425	428
Proved developed reserves		410	387	411	486
Proved developed producing reserves		397	370	393	467
Proved undeveloped reserves		63	38	17	92

[1] Gas reserves are converted to Boe at the rate of six Mcf per Bbl of oil, based upon the approximate relative energy content of gas and oil. This rate is not necessarily indicative of the relationship of natural gas and oil prices. Bitumen and NGL reserves are converted to Boe on a one-to-one basis with oil.