Summary of Changes in AOCI by Component (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 9,116
Other comprehensive income before reclassifications		110	$ 113
Amounts reclassified from AOCI	[1]	18	25
Total other comprehensive income (loss), net of tax		128	138
Ending Balance		9,093
ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard		(3)
CTA
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(3,013)	(3,438)
Other comprehensive income before reclassifications		81	122
Total other comprehensive income (loss), net of tax		81	122
Ending Balance		(2,932)	(3,316)
Pension and other employee benefits
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(981)	(1,122)
Other comprehensive income before reclassifications		37	(2)
Amounts reclassified from AOCI	[1]	15	23
Total other comprehensive income (loss), net of tax		52	21
Ending Balance		(929)	(1,101)
Hedging activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(10)	3
Other comprehensive income before reclassifications		(8)	(6)
Amounts reclassified from AOCI	[1]	3	(1)
Total other comprehensive income (loss), net of tax		(5)	(7)
Ending Balance		(15)	(4)
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		3	1
Other comprehensive income before reclassifications			(1)
Amounts reclassified from AOCI	[1]		3
Total other comprehensive income (loss), net of tax			2
Ending Balance			3
Available-for-sale securities | ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of new accounting standard		(3)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(4,001)	(4,556)
Ending Balance		$ (3,876)	$ (4,418)

[1] See table below for details about these reclassifications.