Contingencies and Commitments - Aggregate Undiscounted Commitments on Gathering Processing and Transportation Agreements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Disclosure Aggregate Undiscounted Commitments On Gathering Processing And Transportation Agreements [Abstract]
Service Agreement Commitment 2013	$ 1,215
Service Agreement Commitment 2014	1,991
Service Agreement Commitment 2015	1,810
Service Agrement Commitment 2016	1,904
Service Agreement Commitment 2017	1,931
Service Agreement Commitment 2018 - 2099	9,381
Service Commitment Agreement Total	$ 18,232