Acquisitions (Details) (USD $)	3 Months Ended		9 Months Ended
	Mar. 31, 2012		Sep. 30, 2011
Total Purchase Price [Abstract]
Unproved Oil and Gas Properties	$ 853,000,000
Proved Oil and Gas Properties	386,000,000
Investment in CONE Gathering LLC	69,000,000
Total Assets Acquired	1,308,000,000	[1]
Ratio of funding on share of future drilling and completion costs	one-third
Interest in joint venture partnership (in hundredths)			50.00%
Undeveloped acres purchased (in acres)			628,000
Approximate purchase price of joint venture partnership			1,300,000,000
Cash paid to joint venture partnership	596,000,000
Remaining annual installments to purchase joint venture partnership	2
Calendar year funding of future drilling and completion costs, maximum (per year)	400,000,000
Future drilling and completion costs, maximum	2,100,000,000
Cost Obligation expected term, minimum (in years)	8 years
Natural gas price agreed upon benchmark, average (in MMBtu)	4.00
Imputed discount on CONSOL installment payments	$ 17,000,000

[1] Total reflects impact of $17 million imputed discount on CONSOL installment payments.