Accumulated Other Comprehensive Income (Loss) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 15,457
Other comprehensive income (loss), before reclassifications	(219)
Reclassifications to Consolidated Statement of Income	84
Balance	15,912
Interest-rate Derivatives Previously Subject to Hedge Accounting [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(42)
Other comprehensive income (loss), before reclassifications	0
Reclassifications to Consolidated Statement of Income	4
Balance	(38)
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(341)
Other comprehensive income (loss), before reclassifications	(219)
Reclassifications to Consolidated Statement of Income	80
Balance	(480)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(383)
Balance	$ (518)