Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Deferred tax assets, Depreciation	$ 194	$ 147
Deferred tax assets, Employee benefits	2,517	2,413
Deferred tax assets, Loss provisions	486	441
Deferred tax assets, Deferred income/expense	37	30
Deferred tax assets, Tax loss carryforwards	1,917	2,075
Deferred tax assets, Tax credit carryforwards	693	625
Deferred tax assets, Derivatives and hedging activities		5
Deferred tax assets, Other	680	521
Deferred tax assets, Gross	6,524	6,257
Deferred tax assets, Valuation allowance	(2,037)	(1,668)	$ (1,804)	$ (1,400)
Deferred tax assets, net	4,487	4,589
Deferred tax liabilities, Depreciation	1,353	1,187
Deferred tax liabilities, Employee benefits	34	37
Deferred tax liabilities, Loss provisions	9	10
Deferred tax liabilities, Deferred income/expense	294	230
Deferred tax liabilities, Tax loss carryforwards	0	0
Deferred tax liabilities, Tax credit carryforwards	0	0
Deferred tax liabilities, Derivatives and hedging activities	276	39
Deferred tax liabilities, Other	339	297
Deferred tax liabilities, Gross	2,305	1,800
Deferred tax liabilities, Valuation allowance	0	0
Deferred tax liabilities, Net	$ 2,305	$ 1,800