Acquisitions and Divestitures - Additional Information - 2014 Activity - Gulf of Mexico Divestiture (Detail) - USD ($) $ in Millions		9 Months Ended
	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of 11 primary-term deepwater exploration blocks		$ 148	$ 390
Gulf of Mexico Divestiture [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of 11 primary-term deepwater exploration blocks	$ 1,400
Effective date of sale	May 01, 2014