Supplemental Disclosures About Oil And Gas Producing Activities (Schedule Of Net Proved Oil And Gas Reserves And Changes In Net Proved Oil And Gas Reserves) (Details)	12 Months Ended
	Dec. 31, 2012 MBbls	Dec. 31, 2011 MBbls	Dec. 31, 2010 MBbls
Crude Oil, Condensate And Natural Gas Liquids [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	34,659,000	28,473,000	14,803,000
Extensions and discoveries	17,153,000	17,404,000	16,224,000
Revisions of previous estimates	2,304,000	103,000	(2,102,000)
Production	(3,167,000)	(1,011,000)	(452,000)
Proved developed and undeveloped reserves end of year	49,699,000	34,659,000	28,473,000
Sales of reserves in place	(1,250,000)	(10,310,000)
Proved developed reserves at beginning of year	10,708,000	7,387,000
Proved developed reserves at end of year	19,536,000	10,708,000	7,387,000
Proved undeveloped reserves at beginning of year	23,951,000	21,086,000
Proved undeveloped reserves at end of year	30,163,000	23,951,000	21,086,000
Crude Oil, Condensate And Natural Gas Liquids [Member] | U.S. [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	29,222,000	23,210,000	14,803,000
Extensions and discoveries	17,153,000	17,404,000	10,961,000
Revisions of previous estimates	2,500,000	(71,000)	(2,102,000)
Production	(3,167,000)	(1,011,000)	(452,000)
Proved developed and undeveloped reserves end of year	44,458,000	29,222,000	23,210,000
Sales of reserves in place	(1,250,000)	(10,310,000)
Proved developed reserves at beginning of year	7,989,000	7,387,000
Proved developed reserves at end of year	14,295,000	7,989,000	7,387,000
Proved undeveloped reserves at beginning of year	21,233,000	15,823,000
Proved undeveloped reserves at end of year	30,163,000	21,233,000	15,823,000
Crude Oil, Condensate And Natural Gas Liquids [Member] | U.K.
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	5,437,000	5,263,000	0
Extensions and discoveries	0	0	5,263,000
Revisions of previous estimates	(196,000)	174,000	0
Production	0	0	0
Proved developed and undeveloped reserves end of year	5,241,000	5,437,000	5,263,000
Sales of reserves in place	0	0
Proved developed reserves at beginning of year	2,719,000	0
Proved developed reserves at end of year	5,241,000	2,719,000	0
Proved undeveloped reserves at beginning of year	2,718,000	5,263,000
Proved undeveloped reserves at end of year	0	2,718,000	5,263,000
Natural Gas [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	727,685,000	669,851,000	513,047,000
Extensions and discoveries	72,916,000	221,544,000	245,031,000
Revisions of previous estimates	(21,170,000)	(41,836,000)	(54,132,000)
Production	(37,612,000)	(38,990,000)	(34,095,000)
Proved developed and undeveloped reserves end of year	428,336,000	727,685,000	669,851,000
Sales of reserves in place	(313,483,000)	(82,884,000)
Proved developed reserves at beginning of year	392,214,000	358,543,000
Proved developed reserves at end of year	234,203,000	392,214,000	358,543,000
Proved undeveloped reserves at beginning of year	335,471,000	311,308,000
Proved undeveloped reserves at end of year	194,134,000	335,471,000	311,308,000
Natural Gas [Member] | U.S. [Member]
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	722,847,000	665,167,000	513,047,000
Extensions and discoveries	72,916,000	221,544,000	240,347,000
Revisions of previous estimates	(20,996,000)	(41,990,000)	(54,132,000)
Production	(37,612,000)	(38,990,000)	(34,095,000)
Proved developed and undeveloped reserves end of year	423,672,000	722,847,000	665,167,000
Sales of reserves in place	(313,483,000)	(82,884,000)
Proved developed reserves at beginning of year	389,795,000	358,543,000
Proved developed reserves at end of year	229,539,000	389,795,000	358,543,000
Proved undeveloped reserves at beginning of year	333,052,000	306,624,000
Proved undeveloped reserves at end of year	194,134,000	333,052,000	306,624,000
Natural Gas [Member] | U.K.
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed and undeveloped reserves beginning of year	4,838,000	4,684,000	0
Extensions and discoveries	0	0	4,684,000
Revisions of previous estimates	(174,000)	154,000	0
Production	0	0	0
Proved developed and undeveloped reserves end of year	4,664,000	4,838,000	4,684,000
Sales of reserves in place	0	0
Proved developed reserves at beginning of year	2,419,000	0
Proved developed reserves at end of year	4,664,000	2,419,000	0
Proved undeveloped reserves at beginning of year	2,419,000	4,684,000
Proved undeveloped reserves at end of year	0	2,419,000	4,684,000