Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 MMcfe Mcf	Dec. 31, 2011 Mcf	Dec. 31, 2010 Mcf
Reserve Quantities [Line Items]
Percentage of proved resources reviewed by consultants	0.93%
Variance in reserve estimates	less than 5%
Scheduled to be drilling period maximum for undeveloped reserves	Scheduled to be drilled within five years
Proved reserves from drilling activities and evaluations of proved areas	1,800,000,000	1,500,000,000	1,400,000,000
Reserve attributable to natural gas	56.00%	87.00%	77.00%
Positive performance revisions for ethane reserves	307,000,000
Revisions of previous estimates	109,000,000	225,000,000	148,600,000
Positive performance revisions for natural gas properties			40,500,000
Cost spent on undeveloped reserves transferred to developed reserves	$ 451.9
Volume of Mmcfe of reserves that have been reported for five or more years included in proved undeveloped reserves, approximately	10,888
Percentage of proved undeveloped reserves	10.00%
Period for which the reserves have been reported to be included in proved undeveloped reserves	Five or more years
Maximum [Member]
Reserve Quantities [Line Items]
Percentage of proved undeveloped reserves	1.00%
Year 2013 [Member]
Reserve Quantities [Line Items]
Estimated future development costs	453.7
Year 2014 [Member]
Reserve Quantities [Line Items]
Estimated future development costs	506.1
Year 2015 [Member]
Reserve Quantities [Line Items]
Estimated future development costs	$ 1,100.0
Crude Oil [Member]
Reserve Quantities [Line Items]
Estimate reserve information average realized prices	86.91	85.59	72.51
Proved reserves from drilling activities and evaluations of proved areas	15,131	4,915	4,663
Revisions of previous estimates	2,316	6,522	(2,672)
NGLs [Member]
Reserve Quantities [Line Items]
Estimate reserve information average realized prices	32.23	49.24	39.14
Proved reserves from drilling activities and evaluations of proved areas	113,392	26,591	48,792
Positive performance revisions for ethane reserves	51,200
Revisions of previous estimates	3,036	18,627	26,832
Natural Gas [Member]
Reserve Quantities [Line Items]
Estimate reserve information average realized prices	2.75	3.55	3.70
Proved reserves from drilling activities and evaluations of proved areas	996,059	1,304,324	1,089,632
Revisions of previous estimates	76,925	73,643	3,599
Crude Oil and NGL [Member]
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices	95.05	95.61	79.81
Natural Gas, Per Thousand Cubic Feet [Member]
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices	2.76	4.12	4.38