Property, Plant and Equipment - Capitalized Exploratory Well Costs - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014 Well
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized exploratory well costs charged to expense	$ 235	$ 267	$ 129
Exploratory dry hole costs which were incurred and subsequently expensed	66	77	248
Capitalized exploratory well costs that have been capitalized for period greater than one year	1,232
Green Canyon 469 Block [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized exploratory well costs charged to expense	169
Area 54, Offshore Libya [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized exploratory well costs charged to expense		$ 260
Block WA-390-P, Offshore Western Australia [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized well costs	68.00%
Offshore Ghana [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized well costs	30.00%
Number of consecutive successful exploration wells	7
Number of wells appraisal drilling plans		7
Number of wells appraisal drilling plans approved	4
Number of wells appraisal drilling plans in process	3
Number of appraisal wells completed				3
Projects under planning and development [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized well costs	2.00%