Natural Gas and Oil Producing Activities (Changes in Proved Natural Gas and Oil Reserves) (Details)	12 Months Ended
	Dec. 31, 2011 MMcf	Dec. 31, 2010 MMcf	Dec. 31, 2009 MMcf
Natural Gas [Member]
Proved reserves, beginning of year	4,929,980	3,650,303	2,175,528
Revisions of previous estimates	34,505	309,292	94,930
Extensions, discoveries and other additions	1,459,428	1,429,439	1,683,264
Production	(499,433)	(403,636)	(299,698)
Acquisition of reserves in place	13	0	1,795
Disposition of reserves in place	(37,286)	(55,418)	(5,516)
Proved reserves, end of year	5,887,207	4,929,980	3,650,303
Proved Developed Reserves Abstract
Beginning of year	2,687,238	1,972,767	1,336,370
End of year	3,254,018	2,687,238	1,972,767
Proved Undeveloped Reserves Abstract
Beginning of year	2,242,742	1,677,536	839,158
End of year	2,633,189	2,242,742	1,677,536
Oil [Member]
Proved reserves, beginning of year	1,219	1,059	1,507
Revisions of previous estimates	(125)	50	(346)
Extensions, discoveries and other additions	2	281	22
Production	(97)	(171)	(124)
Acquisition of reserves in place	0	0	0
Disposition of reserves in place	(3)	0	0
Proved reserves, end of year	996	1,219	1,059
Proved Developed Reserves Abstract
Beginning of year	1,173	1,028	1,352
End of year	983	1,173	1,028
Proved Undeveloped Reserves Abstract
Beginning of year	46	31	155
End of year	13	46	31