Securities - Gross unrealized losses and related fair values (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 security	Dec. 31, 2013 security
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	79	262
Securities available-for-sale gross unrealized losses one year or less	$ (80)	$ (618)
Securities available-for-sale aggregate fair value of investment one year or less	7,745	28,250
Number of Securities available for sale greater than one year	73	53
Securities available-for-sale gross unrealized losses greater than one year	(55)	(109)
Securities available-for-sale aggregate fair value of investment greater than one year	3,441	4,227
Number of securities held-to-maturity in one year or less	247	99
Securities held-to-maturity gross unrealized losses one year or less	(3)	0
Securities held-to-maturity aggregate fair value of investment one year or less	1,379	4
Securities held-to-maturity greater than one year	850	909
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	3
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	6	22
Securities available-for-sale gross unrealized losses one year or less	(47)	(82)
Securities available-for-sale aggregate fair value of investment one year or less	3,459	16,958
Number of Securities available for sale greater than one year	4	6
Securities available-for-sale gross unrealized losses greater than one year	(11)	(31)
Securities available-for-sale aggregate fair value of investment greater than one year	1,546	630
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	23
Securities available-for-sale gross unrealized losses one year or less	(1)	(12)
Securities available-for-sale aggregate fair value of investment one year or less	128	400
Number of Securities available for sale greater than one year	24	20
Securities available-for-sale gross unrealized losses greater than one year	(6)	(19)
Securities available-for-sale aggregate fair value of investment greater than one year	391	356
Number of securities held-to-maturity in one year or less	144	13
Securities held-to-maturity gross unrealized losses one year or less	(1)	0
Securities held-to-maturity aggregate fair value of investment one year or less	394	0
Securities held-to-maturity greater than one year	47	48
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
US 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	30	170
Securities available-for-sale gross unrealized losses one year or less	(20)	(494)
Securities available-for-sale aggregate fair value of investment one year or less	2,046	10,243
Number of Securities available for sale greater than one year	10	5
Securities available-for-sale gross unrealized losses greater than one year	(9)	(2)
Securities available-for-sale aggregate fair value of investment greater than one year	213	23
Number of securities held-to-maturity in one year or less	103	79
Securities held-to-maturity gross unrealized losses one year or less	(2)	0
Securities held-to-maturity aggregate fair value of investment one year or less	985	0
Securities held-to-maturity greater than one year	800	859
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 subdivision [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	34	42
Securities available-for-sale gross unrealized losses one year or less	(2)	(19)
Securities available-for-sale aggregate fair value of investment one year or less	146	330
Number of Securities available for sale greater than one year	23	5
Securities available-for-sale gross unrealized losses greater than one year	(2)	(6)
Securities available-for-sale aggregate fair value of investment greater than one year	194	65
Number of securities held-to-maturity in one year or less	0	7
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	3	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	1	1
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	(6)
Securities available-for-sale aggregate fair value of investment one year or less	3	115
Number of Securities available for sale greater than one year	9	10
Securities available-for-sale gross unrealized losses greater than one year	(24)	(36)
Securities available-for-sale aggregate fair value of investment greater than one year	199	237
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	5	1
Securities available-for-sale gross unrealized losses one year or less	(9)	0
Securities available-for-sale aggregate fair value of investment one year or less	1,805	50
Number of Securities available for sale greater than one year	3	7
Securities available-for-sale gross unrealized losses greater than one year	(3)	(15)
Securities available-for-sale aggregate fair value of investment greater than one year	898	2,916
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less	1	1
Securities available-for-sale gross unrealized losses one year or less	(1)	(5)
Securities available-for-sale aggregate fair value of investment one year or less	158	154
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

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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