Acquisitions and Divestitures - Additional Information - 2014 Activity - Canada Divestiture (Detail) $ in Millions		6 Months Ended
	Apr. 30, 2014 USD ($) a	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of oil and gas assets		$ 119	$ 381
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] | Wapiti Canada [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Working interest percentage	100.00%