Securities - Gross unrealized losses and related fair values (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 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	50	45
Securities available-for-sale gross unrealized losses one year or less	$ (14)	$ (105)
Securities available-for-sale aggregate fair value of investment one year or less	5,022	10,107
Number of Securities available for sale greater than one year	43	54
Securities available-for-sale gross unrealized losses greater than one year	(139)	(245)
Securities available-for-sale aggregate fair value of investment greater than one year	4,742	3,017
Number of securities held-to-maturity in one year or less	102	678
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	5	2
Securities held-to-maturity greater than one year	1,002	482
Securities held-to-maturity gross unrealized losses greater than one year	0	(1)
Securities held-to-maturity aggregate fair value of investment greater than one year	9	17
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	6	5
Securities available-for-sale gross unrealized losses one year or less	(3)	(1)
Securities available-for-sale aggregate fair value of investment one year or less	3,344	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	(21)	(120)
Securities available-for-sale aggregate fair value of investment greater than one year	587	2,592
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	9	6
Securities available-for-sale gross unrealized losses one year or less	(3)	0
Securities available-for-sale aggregate fair value of investment one year or less	431	8
Number of Securities available for sale greater than one year	14	15
Securities available-for-sale gross unrealized losses greater than one year	0	0
Securities available-for-sale aggregate fair value of investment greater than one year	7	9
Number of securities held-to-maturity in one year or less	24	47
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	0	0
Securities held-to-maturity greater than one year	52	11
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	0	0
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	18	14
Securities available-for-sale gross unrealized losses one year or less	(6)	(6)
Securities available-for-sale aggregate fair value of investment one year or less	1,059	833
Number of Securities available for sale greater than one year	0	2
Securities available-for-sale gross unrealized losses greater than one year	0	0
Securities available-for-sale aggregate fair value of investment greater than one year	0	4
Number of securities held-to-maturity in one year or less	75	629
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	0	2
Securities held-to-maturity greater than one year	947	463
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	2	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	14	3
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	168	20
Number of Securities available for sale greater than one year	1	3
Securities available-for-sale gross unrealized losses greater than one year	0	0
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	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	1	0
Securities held-to-maturity greater than one year	1	4
Securities held-to-maturity gross unrealized losses greater than one year	0	0
Securities held-to-maturity aggregate fair value of investment greater than one year	0	2
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	3	2
Securities available-for-sale gross unrealized losses one year or less	0	0
Securities available-for-sale aggregate fair value of investment one year or less	20	45
Number of Securities available for sale greater than one year	13	22
Securities available-for-sale gross unrealized losses greater than one year	(70)	(125)
Securities available-for-sale aggregate fair value of investment greater than one year	354	387
Number of securities held-to-maturity in one year or less	1	0
Securities held-to-maturity gross unrealized losses one year or less	0	0
Securities held-to-maturity aggregate fair value of investment one year or less	4	0
Securities held-to-maturity greater than one year	2	4
Securities held-to-maturity gross unrealized losses greater than one year	0	(1)
Securities held-to-maturity aggregate fair value of investment greater than one year	7	14
Corporate and other domestic debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	0	0
Securities available-for-sale gross unrealized losses one year or less	0	0
Securities available-for-sale aggregate fair value of investment one year or less	0	0
Number of Securities available for sale greater than one year	0	0
Securities available-for-sale gross unrealized losses greater than one year	0	0
Securities available-for-sale aggregate fair value of investment greater than one year	0	0
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	0	15
Securities available-for-sale gross unrealized losses one year or less	0	(97)
Securities available-for-sale aggregate fair value of investment one year or less	0	4,223
Number of Securities available for sale greater than one year	9	0
Securities available-for-sale gross unrealized losses greater than one year	(48)	0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,787	$ 0

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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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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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