Income Taxes - Components of Deferred Tax Liabilities and Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred tax liabilities
Property, plant and equipment and investments	$ (3,743)	$ (4,226)
Other	(257)	(269)
Total deferred tax liabilities	(4,000)	(4,495)
Deferred tax assets
Net operating loss carryforwards	3,852	3,010
Tax credit carryforwards	188	193
Property, plant and equipment and investments	981	1,110
Accrued compensation, deferred credits and other liabilities	492	449
Asset retirement obligations	1,220	1,421
Other	165	261
Total deferred tax assets	6,898	6,444
Valuation allowances	(1,579)	(1,416)
Total deferred tax assets, net of valuation allowances	5,319	5,028
Net deferred tax assets	$ 1,319	$ 533