Capitalized Exploratory Well Costs - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
	Jun. 30, 2013 Well	Mar. 31, 2015 Well	Sep. 30, 2014 Well
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Exploratory dry hole costs which were incurred and subsequently expensed		49
Capitalized exploratory well costs that have been capitalized for period greater than one year		1,217
Dinarta Block, Kurdistan Region of Iraq [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized exploration well and related leasehold costs charges after tax		67
Capitalized exploration well and related leasehold costs charges pre-tax		159
Block WA-390-P, Offshore Western Australia [Member]
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Capitalized well costs		70.00%
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