Acquisitions and Divestitures - Additional Information - 2014 Activity - Canada Divestiture (Detail) $ in Millions		9 Months Ended
	Apr. 30, 2014 USD ($) a	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of oil and gas assets		$ 148	$ 390
Natural Gas [Member] | Canada [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of oil and gas assets	$ 374
Oil and gas properties sold | a	328,400
Effective date of sale	Jan. 01, 2014
Natural Gas Liquids [Member] | Canada [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Working interest percentage in Wapiti area	100.00%