Oil and Gas Operations (Unaudited) (Oil and Gas Operations) (Details)	12 Months Ended
Dec. 31, 2012 MMBoe	Dec. 31, 2011 MMBoe	Dec. 31, 2010 MMBoe
Reserve Quantities [Line Items]
Percentage of proved reserves audited by independent reservoir enginners	99.00%
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period, total	343.1	302.9	257.8
Revisions of previous estimates, total	(42.1)	(5.8)	11.3
Purchases, total	12.4	20.8	25.8
Extensions and discoveries, total	57.1	45.6	26.8
Production, total	(24.1)	(20.4)	(18.8)
Sales, total			0.0
Proved reserves at end of period, total	346.4	343.1	302.9
Proved developed reserves at end of period (BOE)	260.5	248.5	231.9
Proved undeveloped reserves at end of period (BOE)	85.9	94.6	71.0
Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	957,368	954,387	897,546
Revisions of previous estimates	(143,704)	(12,823)	66,679
Purchases	10,656	19,362	21,700
Extensions and discoveries	61,170	68,160	39,570
Production	(76,362)	(71,718)	(70,924)
Sales			(184)
Proved reserves at end of period	809,128	957,368	954,387
Proved developed reserves at end of period (volume)	708,657	788,812	786,292
Proved undeveloped reserves at end of period (volume)	100,471	168,556	168,095
Oil Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	129,578	103,262	77,963
Revisions of previous estimates	(8,546)	(4,513)	(2,243)
Purchases	7,950	12,583	16,443
Extensions and discoveries	35,132	24,564	16,234
Production	(8,766)	(6,318)	(5,131)
Sales			(4)
Proved reserves at end of period	155,348	129,578	103,262
Proved developed reserves at end of period (volume)	105,976	83,899	72,030
Proved undeveloped reserves at end of period (volume)	49,372	45,679	31,232
Natural Gas Liquids Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	53,957	40,601	30,257
Revisions of previous estimates	(9,557)	841	2,434
Purchases	2,569	5,055	5,730
Extensions and discoveries	11,759	9,637	4,058
Production	(2,573)	(2,177)	(1,880)
Sales			2
Proved reserves at end of period	56,155	53,957	40,601
Proved developed reserves at end of period (volume)	36,440	33,154	28,809
Proved undeveloped reserves at end of period (volume)	19,715	20,803	11,792

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Proved Developed And Undeveloped Reserves, Net, Energy

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Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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Production of proved reserves.

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