Income Taxes - Components of Deferred Tax Liabilities and Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Liabilities
Property, plant and equipment and investments	$ (853)	$ (629)
Other	(77)	(24)
Total Deferred Tax Liabilities	(930)	(653)
Deferred Tax Assets
Net operating loss carryforwards	4,239	4,029
Tax credit carryforwards	134	138
Property, plant and equipment and investments	416	746
Accrued compensation, deferred credits and other liabilities	232	283
Asset retirement obligations	225	212
Other	161	36
Total Deferred Tax Assets	5,407	5,444
Valuation allowances	(4,877)	(5,199)
Total deferred tax assets, net of valuation allowances	530	245
Net Deferred Tax Assets (Liabilities)	$ (400)	$ (408)