Oil and Gas Operations (Unaudited) (Oil and Gas Operations) (Details)	12 Months Ended
	Dec. 31, 2011 mmboe	Dec. 31, 2010 mmboe	Dec. 31, 2009 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	302.9	257.8	264.1
Revisions of previous estimates	(5.8)	11.3	(20.7)
Purchases	20.8	25.8	16.2
Extensions and discoveries	45.6	26.8	17.7
Production	(20.4)	(18.8)	(18.5)
Sales		0.0	(1.0)
Proved reserves at end of period	343.1	302.9	257.8
Proved developed reserves at end of period (BOE)	248.5	231.9	215.0
Proved undeveloped reserves at end of period (BOE)	94.6	71.0	42.8
Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	954,387	897,546	1,038,453
Revisions of previous estimates	(12,823)	66,679	(122,862)
Purchases	19,362	21,700	9,646
Extensions and discoveries	68,160	39,570	45,791
Production	(71,718)	(70,924)	(72,337)
Sales		(184)	(1,145)
Proved reserves at end of period	957,368	954,387	897,546
Proved developed reserves at end of period (volume)	788,812	786,292	743,859
Proved undeveloped reserves at end of period (volume)	168,556	168,095	153,687
Oil Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	103,262	77,963	62,034
Revisions of previous estimates	(4,513)	(2,243)	1,175
Purchases	12,583	16,443	12,064
Extensions and discoveries	24,564	16,234	8,144
Production	(6,318)	(5,131)	(4,690)
Sales		(4)	(764)
Proved reserves at end of period	129,578	103,262	77,963
Proved developed reserves at end of period (volume)	83,899	72,030	66,078
Proved undeveloped reserves at end of period (volume)	45,679	31,232	11,885
Natural Gas Liquids Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	40,601	30,257	28,953
Revisions of previous estimates	841	2,434	(1,411)
Purchases	5,055	5,730	2,537
Extensions and discoveries	9,637	4,058	1,969
Production	(2,177)	(1,880)	(1,791)
Sales		2	0
Proved reserves at end of period	53,957	40,601	30,257
Proved developed reserves at end of period (volume)	33,154	28,809	24,985
Proved undeveloped reserves at end of period (volume)	20,803	11,792	5,272