Acquisitions and Divestitures (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Cordillera Energy Partners [Member]
Assets acquired and liabilities assumed as of the acquisition date
Current assets	$ 56
Proved properties	1,040
Unproved properties	2,288
Gathering, transmission and processing facilities	1
Total assets acquired	3,385
Current liabilities	86
Non-current obligations	5
Total liabilities assumed	91
Net assets acquired	3,294
Mobil North Sea Limited [Member]
Assets acquired and liabilities assumed as of the acquisition date
Current assets	208
Proved properties	2,341
Unproved properties	476
Gathering, transmission and processing facilities	338
Goodwill	82
Total assets acquired	3,445
Current liabilities	148
Asset retirement obligation	517
Deferred income tax liabilities	1,533
Other long-term obligations	1
Total liabilities assumed	2,199
Net assets acquired	$ 1,246