Securities - Gross unrealized losses and related fair values (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012 Security	Dec. 31, 2011 Security
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	37	45
Securities available-for-sale gross unrealized losses one year or less	$ (16)	$ (105)
Securities available-for-sale aggregate fair value of investment one year or less	4,480	10,107
Number of Securities available for sale greater than one year	40	54
Securities available-for-sale gross unrealized losses greater than one year	(174)	(245)
Securities available-for-sale aggregate fair value of investment greater than one year	3,924	3,017
Number of securities held-to-maturity in one year or less	71	678
Securities held-to-maturity gross unrealized losses one year or less
Securities held-to-maturity aggregate fair value of investment one year or less	4	2
Securities held-to-maturity greater than one year	1,025	482
Securities held-to-maturity gross unrealized losses greater than one year		(1)
Securities held-to-maturity aggregate fair value of investment greater than one year	11	17
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	2	6
Securities available-for-sale gross unrealized losses one year or less	(2)
Securities available-for-sale aggregate fair value of investment one year or less	231	8
Number of Securities available for sale greater than one year	12	15
Securities available-for-sale gross unrealized losses greater than one year
Securities available-for-sale aggregate fair value of investment greater than one year	8	9
Number of securities held-to-maturity in one year or less	12	47
Securities held-to-maturity gross unrealized losses one year or less
Securities held-to-maturity aggregate fair value of investment one year or less
Securities held-to-maturity greater than one year	52	11
Securities held-to-maturity gross unrealized losses greater than one year
U.S. Government agency issued or guaranteed [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	19	14
Securities available-for-sale gross unrealized losses one year or less	(4)	(6)
Securities available-for-sale aggregate fair value of investment one year or less	116	833
Number of Securities available for sale greater than one year		2
Securities available-for-sale gross unrealized losses greater than one year
Securities available-for-sale aggregate fair value of investment greater than one year		4
Number of securities held-to-maturity in one year or less	56	629
Securities held-to-maturity gross unrealized losses one year or less
Securities held-to-maturity aggregate fair value of investment one year or less		2
Securities held-to-maturity greater than one year	969	463
Securities held-to-maturity gross unrealized losses greater than one year
Securities held-to-maturity aggregate fair value of investment greater than one year	3	1
Obligations of U.S. states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	4	3
Securities available-for-sale gross unrealized losses one year or less		(1)
Securities available-for-sale aggregate fair value of investment one year or less	29	20
Number of Securities available for sale greater than one year	1	3
Securities available-for-sale gross unrealized losses greater than one year
Securities available-for-sale aggregate fair value of investment greater than one year	7	25
Number of securities held-to-maturity in one year or less	2	2
Securities held-to-maturity gross unrealized losses one year or less
Securities held-to-maturity greater than one year	2	4
Securities held-to-maturity gross unrealized losses greater than one year
Securities held-to-maturity aggregate fair value of investment greater than one year	1	2
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	4	2
Securities available-for-sale aggregate fair value of investment one year or less	54	45
Number of Securities available for sale greater than one year	13	22
Securities available-for-sale gross unrealized losses greater than one year	(97)	(125)
Securities available-for-sale aggregate fair value of investment greater than one year	339	387
Number of securities held-to-maturity in one year or less	1
Securities held-to-maturity gross unrealized losses one year or less
Securities held-to-maturity aggregate fair value of investment one year or less	4
Securities held-to-maturity greater than one year	2	4
Securities held-to-maturity gross unrealized losses greater than one year		(1)
Securities held-to-maturity aggregate fair value of investment greater than one year	7	14
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	3	5
Securities available-for-sale gross unrealized losses one year or less	(2)	(1)
Securities available-for-sale aggregate fair value of investment one year or less	2,595	4,978
Number of Securities available for sale greater than one year	6	12
Securities available-for-sale gross unrealized losses greater than one year	(28)	(120)
Securities available-for-sale aggregate fair value of investment greater than one year	591	2,592
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	5	15
Securities available-for-sale gross unrealized losses one year or less	(8)	(97)
Securities available-for-sale aggregate fair value of investment one year or less	1,455	4,223
Number of Securities available for sale greater than one year	8
Securities available-for-sale gross unrealized losses greater than one year	(49)
Securities available-for-sale aggregate fair value of investment greater than one year	2,979
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less
Securities available-for-sale gross unrealized losses one year or less
Securities available-for-sale aggregate fair value of investment one year or less
Number of Securities available for sale greater than one year
Securities available-for-sale gross unrealized losses greater than one year
Securities available-for-sale aggregate fair value of investment greater than one year

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

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Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for securities categorized neither as held-to-maturity nor trading securities.

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