Summary of Changes in AOCI by Component (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				$ 8,280
Other comprehensive income before reclassifications				458	$ 77
Amounts reclassified from AOCI	[1]			104	(4,473)
Total other comprehensive income (loss), net of tax		$ 182	$ 32	562	(4,396)
Ending Balance		9,459		9,459
CTA
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				(3,438)	(3,191)
Other comprehensive income before reclassifications				501	(16)
Amounts reclassified from AOCI	[1]			29
Total other comprehensive income (loss), net of tax				530	(16)
Ending Balance		(2,908)	(3,207)	(2,908)	(3,207)
Pension and other employee benefits
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				(1,161)	(1,064)
Other comprehensive income before reclassifications				(25)	(6)
Amounts reclassified from AOCI	[1]			69	67
Total other comprehensive income (loss), net of tax				44	61
Ending Balance		(1,117)	(1,003)	(1,117)	(1,003)
Hedging activities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				3	7
Other comprehensive income before reclassifications				(19)	(6)
Amounts reclassified from AOCI	[1]			3	(4)
Total other comprehensive income (loss), net of tax				(16)	(10)
Ending Balance		(13)	(3)	(13)	(3)
Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				40	4,472
Other comprehensive income before reclassifications				1	105
Amounts reclassified from AOCI	[1]			3	(4,536)
Total other comprehensive income (loss), net of tax				4	(4,431)
Ending Balance		44	41	44	41
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance				(4,556)	224
Ending Balance		$ (3,994)	$ (4,172)	$ (3,994)	$ (4,172)

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