Income Taxes - Components of Deferred Tax Liabilities and Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Liabilities
Property, plant and equipment and investments	$ (629)	$ (3,810)
Other	(24)	(255)
Total Deferred Tax Liabilities	(653)	(4,065)
Deferred Tax Assets
Net operating loss carryforwards	4,029	5,767
Tax credit carryforwards	138	164
Property, plant and equipment and investments	746	834
Accrued compensation, deferred credits and other liabilities	283	526
Asset retirement obligations	212	1,077
Other	36	62
Total Deferred Tax Assets	5,444	8,430
Valuation allowances	(5,199)	(5,450)
Total deferred tax assets, net of valuation allowances	245	2,980
Net Deferred Tax Assets (Liabilities)	$ (408)	$ (1,085)