Aggregate Undiscounted Commitments on Gathering Processing and Transportation Agreements (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Disclosure Aggregate Undiscounted Commitments On Gathering Processing And Transportation Agreements [Abstract]
2012	$ 282
2013	1,194
2014	1,535
2015	1,626
2016	1,683
2017 - 2099	11,235
Total	$ 17,555