Oil and Gas Exploration and Production Industries Disclosures (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 Boe	Dec. 31, 2011 Boe	Dec. 31, 2010 Boe	Dec. 31, 2009 Boe
Proved Developed And Undeveloped Reserves (MBoe) [Rollforward]
Net proved reserves - beginning of period (MBoe)	2,053,763	1,949,535	1,796,019
Revisions of previous estimates (MBoe)	(392,621)	(59,163)	(38,404)
Purchases in place (MBoe)	4,098	521	14
Extensions, discoveries and other additions (MBoe)	406,932	387,255	395,984
Sales in place (MBoe)	(90,420)	(68,247)	(60,148)
Production (MBoe)	(171,054)	(156,138)	(143,930)
Net proved reserves - end of period (MBoe)	1,810,698	2,053,763	1,949,535
Net Proved Developed and Undeveloped Reserves [Abstract]
Net Proved Developed Reserves (BOE)	949,819	1,042,713	1,012,987	976,374
Net Proved Undeveloped Reserve (BOE)	860,879	1,011,050	936,548	819,646
Revision of Proved Reserves Due to Decrease or Increase in the Average Natural Gas Price for the Year as Compared to Prior Year (stated in MMBoe)	(531)	(16)	28
Revisions of Proved Undeveloped Reserves (stated in MMBoe)	(293)	(7)	(12)
Sales in Place of Proved Undeveloped Reserves (stated in MMBoe)	19	9	8
Proved Undeveloped Reserves Drilled and Transferred to Proved Developed Reserves (stated in MMBoe)	138	144	118
Capitalized Costs Related to Proved Undeveloped Reserves Drilled and Transferred to Proved Developed Reserves	$ 2,764,000,000	$ 1,619,000,000	$ 1,280,000,000
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Proved properties	36,872,434,000	32,353,380,000
Unproved properties	1,253,864,000	1,311,055,000
Total	38,126,298,000	33,664,435,000
Accumulated depreciation, depletion and amortization	(16,849,068,000)	(13,981,143,000)
Net capitalized costs	$ 21,277,230,000	$ 19,683,292,000
Crude Oil (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	517,493	385,922	219,539
Revisions of previous estimates	1,688	(25,756)	(9,151)
Purchases in place	1,010	9	13
Extensions, discoveries and other additions	255,686	203,001	204,476
Sales in place	(17,264)	(4,301)	(1,671)
Production	(57,795)	(41,382)	(27,284)
Net proved reserves - end of period	700,818	517,493	385,922
Natural Gas Liquids (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	227,788	151,909	93,461
Revisions of previous estimates	47,856	36,042	27,294
Purchases in place	612	17	0
Extensions, discoveries and other additions	71,574	65,288	42,242
Sales in place	(7,377)	(10,008)	(8)
Production	(20,490)	(15,460)	(11,080)
Net proved reserves - end of period	319,963	227,788	151,909
Natural Gas (Bcf) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	7,850.9	8,470.2	8,898.1
Revisions of previous estimates	(2,653)	(416.7)	(339.3)
Purchases in place	14.8	3.0	0
Extensions, discoveries and other additions	478.1	713.8	895.6
Sales in place	(394.7)	(323.6)	(350.8)
Production	(556.6)	(595.8)	(633.4)
Net proved reserves - end of period	4,739.5	7,850.9	8,470.2
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	2,979.5	4,155.6	4,457.8	4,633.2
Net proved undeveloped reserves	1,760.0	3,695.3	4,012.4	4,264.9
Liquids (MBbl) [Member]
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	453,242	350,118	270,017	204,177
Net proved undeveloped reserves	567,538	395,163	267,814	108,823
United States [Member]
Proved Developed And Undeveloped Reserves (MBoe) [Rollforward]
Net proved reserves - beginning of period (MBoe)	1,729,508	1,587,806	1,338,292
Revisions of previous estimates (MBoe)	(237,936)	(42,526)	(17,945)
Purchases in place (MBoe)	4,098	521	14
Extensions, discoveries and other additions (MBoe)	392,196	373,602	378,582
Sales in place (MBoe)	(87,588)	(68,247)	(6,860)
Production (MBoe)	(138,170)	(121,648)	(104,277)
Net proved reserves - end of period (MBoe)	1,662,108	1,729,508	1,587,806
Net Proved Developed and Undeveloped Reserves [Abstract]
Net Proved Developed Reserves (BOE)	840,564	877,301	839,928	744,339
Net Proved Undeveloped Reserve (BOE)	821,544	852,207	747,878	593,953
United States [Member] | Crude Oil (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	495,296	355,457	188,452
Revisions of previous estimates	4,105	(21,188)	(8,313)
Purchases in place	1,010	9	13
Extensions, discoveries and other additions	241,171	202,552	199,479
Sales in place	(15,921)	(4,301)	(1,082)
Production	(54,632)	(37,233)	(23,092)
Net proved reserves - end of period	671,029	495,296	355,457
United States [Member] | Natural Gas Liquids (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	226,586	150,434	91,489
Revisions of previous estimates	47,293	35,999	27,490
Purchases in place	612	17	0
Extensions, discoveries and other additions	71,396	65,288	42,221
Sales in place	(7,300)	(10,008)	(2)
Production	(20,181)	(15,144)	(10,764)
Net proved reserves - end of period	318,406	226,586	150,434
United States [Member] | Natural Gas (Bcf) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	6,045.8	6,491.5	6,350.1
Revisions of previous estimates	(1,736)	(344)	(222.7)
Purchases in place	14.8	3.0	0
Extensions, discoveries and other additions	477.8	634.6	821.3
Sales in place	(386.2)	(323.6)	(34.6)
Production	(380.2)	(415.7)	(422.6)
Net proved reserves - end of period	4,036.0	6,045.8	6,491.5
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	2,387.5	3,234.9	3,519.7	3,330.1
Net proved undeveloped reserves	1,648.5	2,810.8	2,971.7	3,020.0
United States [Member] | Liquids (MBbl) [Member]
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	442,648	338,144	253,308	189,322
Net proved undeveloped reserves	546,786	383,739	252,583	90,619
Canada [Member]
Proved Developed And Undeveloped Reserves (MBoe) [Rollforward]
Net proved reserves - beginning of period (MBoe)	192,448	216,084	285,808
Revisions of previous estimates (MBoe)	(151,015)	(12,865)	(5,288)
Purchases in place (MBoe)	0	0	0
Extensions, discoveries and other additions (MBoe)	5,860	448	3,789
Sales in place (MBoe)	(2,832)	0	(53,288)
Production (MBoe)	(8,657)	(11,219)	(14,937)
Net proved reserves - end of period (MBoe)	35,804	192,448	216,084
Net Proved Developed and Undeveloped Reserves [Abstract]
Net Proved Developed Reserves (BOE)	24,348	58,524	79,701	124,323
Net Proved Undeveloped Reserve (BOE)	11,456	133,924	136,383	161,486
Canada [Member] | Crude Oil (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	18,592	25,636	25,586
Revisions of previous estimates	(2,493)	(4,611)	(104)
Purchases in place	0	0	0
Extensions, discoveries and other additions	5,681	449	3,198
Sales in place	(1,343)	0	(589)
Production	(2,574)	(2,882)	(2,455)
Net proved reserves - end of period	17,863	18,592	25,636
Canada [Member] | Natural Gas Liquids (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	1,202	1,475	1,972
Revisions of previous estimates	563	43	(196)
Purchases in place	0	0	0
Extensions, discoveries and other additions	178	0	21
Sales in place	(77)	0	(6)
Production	(309)	(316)	(316)
Net proved reserves - end of period	1,557	1,202	1,475
Canada [Member] | Natural Gas (Bcf) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	1,035.9	1,133.8	1,549.5
Revisions of previous estimates	(894.5)	(49.8)	(29.9)
Purchases in place	0	0	0
Extensions, discoveries and other additions	0	0	3.4
Sales in place	(8.5)	0	(316.2)
Production	(34.6)	(48.1)	(73)
Net proved reserves - end of period	98.3	1,035.9	1,133.8
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	98.3	295.8	401.6	681.0
Net proved undeveloped reserves	0	740.1	732.2	868.5
Canada [Member] | Liquids (MBbl) [Member]
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	7,963	9,220	12,758	10,831
Net proved undeveloped reserves	11,456	10,574	14,352	16,727
Trinidad [Member]
Proved Developed And Undeveloped Reserves (MBoe) [Rollforward]
Net proved reserves - beginning of period (MBoe)	128,629	142,669	169,747
Revisions of previous estimates (MBoe)	(3,953)	(4,011)	(15,513)
Purchases in place (MBoe)	0	0	0
Extensions, discoveries and other additions (MBoe)	0	12,455	12,250
Sales in place (MBoe)	0	0	0
Production (MBoe)	(23,616)	(22,484)	(23,815)
Net proved reserves - end of period (MBoe)	101,060	128,629	142,669
Net Proved Developed and Undeveloped Reserves [Abstract]
Net Proved Developed Reserves (BOE)	81,826	103,710	90,382	105,540
Net Proved Undeveloped Reserve (BOE)	19,234	24,919	52,287	64,207
Trinidad [Member] | Crude Oil (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	3,507	4,731	5,443
Revisions of previous estimates	71	18	(754)
Purchases in place	0	0	0
Extensions, discoveries and other additions	0	0	1,751
Sales in place	0	0	0
Production	(550)	(1,242)	(1,709)
Net proved reserves - end of period	3,028	3,507	4,731
Trinidad [Member] | Natural Gas Liquids (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	0	0	0
Revisions of previous estimates	0	0	0
Purchases in place	0	0	0
Extensions, discoveries and other additions	0	0	0
Sales in place	0	0	0
Production	0	0	0
Net proved reserves - end of period	0	0	0
Trinidad [Member] | Natural Gas (Bcf) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	750.7	827.6	985.8
Revisions of previous estimates	(24.1)	(24.2)	(88.6)
Purchases in place	0	0	0
Extensions, discoveries and other additions	0	74.7	63.0
Sales in place	0	0	0
Production	(138.4)	(127.4)	(132.6)
Net proved reserves - end of period	588.2	750.7	827.6
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	476.7	606.3	519.2	609.4
Net proved undeveloped reserves	111.5	144.4	308.5	376.4
Trinidad [Member] | Liquids (MBbl) [Member]
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	2,378	2,657	3,853	3,966
Net proved undeveloped reserves	651	850	879	1,477
Other International [Member]
Proved Developed And Undeveloped Reserves (MBoe) [Rollforward]
Net proved reserves - beginning of period (MBoe)	3,178	2,976	2,172
Revisions of previous estimates (MBoe)	283	239	342
Purchases in place (MBoe)	0	0	0
Extensions, discoveries and other additions (MBoe)	8,876	750	1,363
Sales in place (MBoe)	0	0	0
Production (MBoe)	(611)	(787)	(901)
Net proved reserves - end of period (MBoe)	11,726	3,178	2,976
Net Proved Developed and Undeveloped Reserves [Abstract]
Net Proved Developed Reserves (BOE)	3,081	3,178	2,976	2,172
Net Proved Undeveloped Reserve (BOE)	8,645	0	0	0
Other International [Member] | Crude Oil (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	98	98	58
Revisions of previous estimates	5	25	20
Purchases in place	0	0	0
Extensions, discoveries and other additions	8,834	0	48
Sales in place	0	0	0
Production	(39)	(25)	(28)
Net proved reserves - end of period	8,898	98	98
Other International [Member] | Natural Gas Liquids (MBbl) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	0	0	0
Revisions of previous estimates	0	0	0
Purchases in place	0	0	0
Extensions, discoveries and other additions	0	0	0
Sales in place	0	0	0
Production	0	0	0
Net proved reserves - end of period	0	0	0
Other International [Member] | Natural Gas (Bcf) [Member]
Proved Developed and Undeveloped Reserves [Rollforward]
Net proved reserves - beginning of period	18.5	17.3	12.7
Revisions of previous estimates	1.6	1.3	1.9
Purchases in place	0	0	0
Extensions, discoveries and other additions	0.3	4.5	7.9
Sales in place	0	0	0
Production	(3.4)	(4.6)	(5.2)
Net proved reserves - end of period	17.0	18.5	17.3
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	17.0	18.6	17.3	12.7
Net proved undeveloped reserves	0	0	0	0
Other International [Member] | Liquids (MBbl) [Member]
Net Proved Developed and Undeveloped Reserves [Abstract]
Net proved developed reserves	253	97	98	58
Net proved undeveloped reserves	8,645	0	0	0