Summary of Changes in AOCI by Component (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		$ 8,280
Other comprehensive income before reclassifications		113	$ 110
Amounts reclassified from AOCI	[1],[2]	25	(3,369)
Total other comprehensive income (loss), net of tax		138	(3,259)
Ending Balance		8,675
CTA
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(3,438)	(3,191)
Other comprehensive income before reclassifications		122	92
Amounts reclassified from AOCI	[2]	0
Total other comprehensive income (loss), net of tax		122	92
Ending Balance		(3,316)	(3,099)
Pension and other employee benefits
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(1,161)	(1,064)
Other comprehensive income before reclassifications		(2)	(1)
Amounts reclassified from AOCI	[1],[2]	23	22
Total other comprehensive income (loss), net of tax		21	21
Ending Balance		(1,140)	(1,043)
Hedging activities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		3	7
Other comprehensive income before reclassifications		(6)	(3)
Amounts reclassified from AOCI	[2]	(1)	(3)
Total other comprehensive income (loss), net of tax		(7)	(6)
Ending Balance		(4)	1
Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		40	4,472
Other comprehensive income before reclassifications		(1)	22
Amounts reclassified from AOCI	[2]	3	(3,388)
Total other comprehensive income (loss), net of tax		2	(3,366)
Ending Balance		42	1,106
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(4,556)	224
Ending Balance		$ (4,418)	$ (3,035)

[1] Amounts in parentheses indicate reductions to net income.

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