SUPPLEMENTAL FINANCIAL DATA-OIL AND GAS PRODUCING ACTIVITIES (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows
Future oil, natural gas, and natural gas liquids sales	$ 6,929,652	$ 11,004,080	$ 9,934,741
Future production costs	(2,166,681)	(2,892,047)	(2,738,345)
Future development costs	(1,444,144)	(2,027,516)	(1,479,932)
Future income taxes	(142,383)	(1,071,362)	(1,120,588)
Future net cash flows	3,176,444	5,013,155	4,595,876
10% annual discount for estimated timing of cash flows	(1,779,347)	(2,625,414)	(2,425,430)
Standardized measure of discounted future net cash flows	1,397,097	2,387,741	2,170,446
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at beginning of year	2,387,741	2,170,446	1,554,309
Changes resulting from:
Sales of oil, natural gas, and NGL net of production costs	(448,614)	(550,013)	(558,400)
Net changes in prices and future production costs	(1,235,758)	118,714	605,043
Net changes in future development costs	(4,188)	(56,422)	(11,597)
Extensions, discoveries, and improved recovery	572,516	667,323	445,546
Development costs incurred during the period	140,111	231,270	134,451
Revisions of previous quantity estimates	(355,565)	(220,389)	48,960
Changes in production rates, timing, and other	(34,665)	(173,222)	50,700
Sales of reserves in place	(213,683)	(107,742)	(34,108)
Purchases of reserves in place	0	0	6,530
Accretion of discount on reserves at beginning of year	263,316	258,303	172,354
Net change in income taxes	325,886	49,473	(243,342)
Total change for year	(990,644)	217,295	616,137
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at end of year	1,397,097	2,387,741	2,170,446
Discount rate applied to cash flow amounts in computation of standardized measure of discounted future net cash flows (as a percent)	10.00%
United States
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows
Future oil, natural gas, and natural gas liquids sales	6,929,652	10,427,716	9,029,839
Future production costs	(2,166,681)	(2,692,993)	(2,546,332)
Future development costs	(1,444,144)	(2,008,824)	(1,462,832)
Future income taxes	(142,383)	(940,526)	(860,047)
Future net cash flows	3,176,444	4,785,373	4,160,628
10% annual discount for estimated timing of cash flows	(1,779,347)	(2,499,631)	(2,195,708)
Standardized measure of discounted future net cash flows	1,397,097	2,285,742	1,964,920
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at beginning of year	2,285,742	1,964,920	1,328,889
Changes resulting from:
Sales of oil, natural gas, and NGL net of production costs	(448,614)	(550,013)	(558,400)
Net changes in prices and future production costs	(1,226,494)	272,027	603,003
Net changes in future development costs	(4,188)	(55,725)	(29,183)
Extensions, discoveries, and improved recovery	572,516	667,323	445,546
Development costs incurred during the period	140,111	231,270	134,451
Revisions of previous quantity estimates	(203,987)	(220,389)	48,960
Changes in production rates, timing, and other	(34,665)	(132,714)	115,768
Sales of reserves in place	(213,683)	(107,742)	(34,108)
Purchases of reserves in place	0	0	6,530
Accretion of discount on reserves at beginning of year	259,393	226,354	139,179
Net change in income taxes	270,966	(9,569)	(235,715)
Total change for year	(888,645)	320,822	636,031
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at end of year	1,397,097	2,285,742	1,964,920
Italy
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows
Future oil, natural gas, and natural gas liquids sales	0	576,364	904,902
Future production costs	0	(199,054)	(192,013)
Future development costs	0	(18,692)	(17,100)
Future income taxes	0	(130,836)	(260,541)
Future net cash flows	0	227,782	435,248
10% annual discount for estimated timing of cash flows	0	(125,783)	(229,722)
Standardized measure of discounted future net cash flows	0	101,999	205,526
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at beginning of year	101,999	205,526	225,420
Changes resulting from:
Sales of oil, natural gas, and NGL net of production costs	0	0	0
Net changes in prices and future production costs	(9,264)	(153,313)	2,040
Net changes in future development costs	0	(697)	17,586
Extensions, discoveries, and improved recovery	0	0	0
Development costs incurred during the period	0	0	0
Revisions of previous quantity estimates	(151,578)	0	0
Changes in production rates, timing, and other	0	(40,508)	(65,068)
Sales of reserves in place	0	0	0
Purchases of reserves in place	0	0	0
Accretion of discount on reserves at beginning of year	3,923	31,949	33,175
Net change in income taxes	54,920	59,042	(7,627)
Total change for year	(101,999)	(103,527)	(19,894)
Standardized measure of discounted future net cash flows relating to proved oil, natural gas, and NGL reserves, at end of year	$ 0	$ 101,999	$ 205,526
Henry Hub
Changes resulting from:
Average price used in calculating standardized measure of discounted future net cash flows proved reserves	2.76	4.12	4.38
West Texas Intermediate
Changes resulting from:
Average price used in calculating standardized measure of discounted future net cash flows proved reserves	94.79	96.08	79.81