Acquisitions and Divestitures - Additional Information - 2014 Activity - Anadarko Basin and Southern Louisiana Divestitures (Detail) $ in Millions	1 Months Ended	9 Months Ended
	Dec. 31, 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]
Proceeds from sale of oil and gas assets		$ 148	$ 390
Anadarko Basin and Southern Louisiana Divestitures [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of oil and gas assets	$ 1,300
Effective date of sale	Oct. 01, 2014
Anadarko Basin [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Oil and gas properties sold | a	115,000
Southern Louisiana [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Oil and gas properties sold | a	90,000