Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 22,318	$ 23,650	$ 21,882
Other comprehensive income (loss), before reclassifications	87	(256)	264
Reclassifications to Consolidated Statement of Income	47	24	91
Net other comprehensive income (loss)	134	(232)	355
Balance	15,457	22,318	23,650
Interest-rate Derivatives Previously Subject to Hedge Accounting [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(48)	(54)	(61)
Other comprehensive income (loss), before reclassifications	0	0	0
Reclassifications to Consolidated Statement of Income	6	6	7
Net other comprehensive income (loss)	6	6	7
Balance	(42)	(48)	(54)
Pension and Other Postretirement Plans [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(469)	(231)	(579)
Other comprehensive income (loss), before reclassifications	87	(256)	264
Reclassifications to Consolidated Statement of Income	41	18	84
Net other comprehensive income (loss)	128	(238)	348
Balance	(341)	(469)	(231)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(517)	(285)	(640)
Balance	$ (383)	$ (517)	$ (285)