Natural Gas and Oil Producing Activities (Unaudited) (Changes in Proved Natural Gas and Oil Reserves) (Details)	12 Months Ended
	Dec. 31, 2012 MMcf	Dec. 31, 2011 MMcf	Dec. 31, 2010 MMcf
Natural Gas [Member]
Proved reserves, beginning of year	5,887,207	4,929,980	3,650,303
Revisions of previous estimates	(2,087,985)	34,505	309,292
Extensions, discoveries and other additions	918,594	1,459,428	1,429,439
Production	(564,484)	(499,433)	(403,636)
Acquisition of reserves in place		13
Disposition of reserves in place	(136,534)	(37,286)	(55,418)
Proved reserves, end of year	4,016,798	5,887,207	4,929,980
Proved Developed Reserves [Abstract]
Beginning of year	3,254,018	2,687,238	1,972,767
End of year	3,195,662	3,254,018	2,687,238
Proved Undeveloped Reserves [Abstract]
Beginning of year	2,633,189	2,242,742	1,677,536
End of year	821,136	2,633,189	2,242,742
Oil [Member]
Proved reserves, beginning of year	996	1,219	1,059
Revisions of previous estimates	(44)	(125)	50
Extensions, discoveries and other additions	154	2	281
Production	(83)	(97)	(171)
Disposition of reserves in place	(779)	(3)
Proved reserves, end of year	244	996	1,219
Proved Developed Reserves [Abstract]
Beginning of year	983	1,173	1,028
End of year	243	983	1,173
Proved Undeveloped Reserves [Abstract]
Beginning of year	13	46	31
End of year	1	13	46