Summary of Significant Accounting Policies - Non-Cash Impairments of Proved and Unproved Property and Equipment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Oil and Gas Properties, Proved [Member]
Schedule Of Significant Accounting Policies [Line Items]
Impairment of Oil and Gas properties	$ 427	$ 7,389	$ 6,068
Oil and Gas Properties, Unproved [Member]
Schedule Of Significant Accounting Policies [Line Items]
Impairment of Oil and Gas properties	$ 272	$ 2,462	$ 1,976