Securities - Gross unrealized losses and related fair values (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 security	Dec. 31, 2012 security
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	71	50
Securities available-for-sale gross unrealized losses one year or less	$ (41)	$ (14)
Securities available-for-sale aggregate fair value of investment one year or less	3,919	5,022
Number of Securities available for sale greater than one year	44	43
Securities available-for-sale gross unrealized losses greater than one year	(113)	(139)
Securities available-for-sale aggregate fair value of investment greater than one year	4,667	4,742
Number of securities held-to-maturity in one year or less	102	102
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	2	5
Securities held-to-maturity greater than one year	966	1,002
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	3	9
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	4	6
Securities available-for-sale gross unrealized losses one year or less	(7)	(3)
Securities available-for-sale aggregate fair value of investment one year or less	686	3,344
Number of Securities available for sale greater than one year	6	6
Securities available-for-sale gross unrealized losses greater than one year	(19)	(21)
Securities available-for-sale aggregate fair value of investment greater than one year	568	587
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	13	9
Securities available-for-sale gross unrealized losses one year or less	(4)	(3)
Securities available-for-sale aggregate fair value of investment one year or less	468	431
Number of Securities available for sale greater than one year	12	14
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	7
Number of securities held-to-maturity in one year or less	20	24
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	48	52
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	39	18
Securities available-for-sale gross unrealized losses one year or less	(27)	(6)
Securities available-for-sale aggregate fair value of investment one year or less	2,593	1,059
Number of Securities available for sale greater than one year	1	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
Number of securities held-to-maturity in one year or less	78	75
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	916	947
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	2
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	14
Securities available-for-sale gross unrealized losses one year or less	(3)	(2)
Securities available-for-sale aggregate fair value of investment one year or less	171	168
Number of Securities available for sale greater than one year	1	1
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	6	7
Number of securities held-to-maturity in one year or less	4	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	2	1
Securities held-to-maturity greater than one year	2	1
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	1	0
Asset backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	1	3
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	1	20
Number of Securities available for sale greater than one year	15	13
Securities available-for-sale gross unrealized losses greater than one year	(61)	(70)
Securities available-for-sale aggregate fair value of investment greater than one year	353	354
Number of securities held-to-maturity in one year or less	0	1
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	4
Securities held-to-maturity greater than one year	0	2
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	7
Foreign 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	9	9
Securities available-for-sale gross unrealized losses greater than one year	(33)	(48)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,733	$ 3,787

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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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