Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Sep. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less \| security	248	79
Securities available-for-sale gross unrealized losses one year or less	$ (314)	$ (80)
Securities available-for-sale aggregate fair value of investment one year or less	$ 17,128	$ 7,745
Number of Securities available for sale greater than one year \| security	52	73
Securities available-for-sale gross unrealized losses greater than one year	$ (53)	$ (55)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 1,189	$ 3,441
Number of securities held-to-maturity in one year or less \| security	231	247
Securities held-to-maturity gross unrealized losses one year or less	$ (9)	$ (3)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,615	$ 1,379
Securities held-to-maturity greater than one year \| security	726	850
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	$ 5	$ 3
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less \| security	33	6
Securities available-for-sale gross unrealized losses one year or less	$ (162)	$ (47)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,187	$ 3,459
Number of Securities available for sale greater than one year \| security	4	4
Securities available-for-sale gross unrealized losses greater than one year	$ (15)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 358	$ 1,546
US Government sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less \| security	149	2
Securities available-for-sale gross unrealized losses one year or less	$ (19)	$ (1)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,137	$ 128
Number of Securities available for sale greater than one year \| security	18	24
Securities available-for-sale gross unrealized losses greater than one year	$ (3)	$ (6)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 192	$ 391
Number of securities held-to-maturity in one year or less \| security	138	144
Securities held-to-maturity gross unrealized losses one year or less	$ (5)	$ (1)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 564	$ 394
Securities held-to-maturity greater than one year \| security	47	47
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 \| security	41	30
Securities available-for-sale gross unrealized losses one year or less	$ (106)	$ (20)
Securities available-for-sale aggregate fair value of investment one year or less	$ 8,061	$ 2,046
Number of Securities available for sale greater than one year \| security	3	10
Securities available-for-sale gross unrealized losses greater than one year	$ (7)	$ (9)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 106	$ 213
Number of securities held-to-maturity in one year or less \| security	93	103
Securities held-to-maturity gross unrealized losses one year or less	$ (4)	$ (2)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,051	$ 985
Securities held-to-maturity greater than one year \| security	676	800
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	$ 4	$ 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 \| security	17	34
Securities available-for-sale gross unrealized losses one year or less	$ (2)	$ (2)
Securities available-for-sale aggregate fair value of investment one year or less	$ 173	$ 146
Number of Securities available for sale greater than one year \| security	17	23
Securities available-for-sale gross unrealized losses greater than one year	$ (2)	$ (2)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 156	$ 194
Number of securities held-to-maturity in one year or less \| security	0	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	$ 0	$ 0
Securities held-to-maturity greater than one year \| security	3	3
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 \| security	0	1
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	$ 3
Number of Securities available for sale greater than one year \| security	9	9
Securities available-for-sale gross unrealized losses greater than one year	$ (26)	$ (24)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 185	$ 199
Foreign debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less \| security	7	5
Securities available-for-sale gross unrealized losses one year or less	$ (24)	$ (9)
Securities available-for-sale aggregate fair value of investment one year or less	$ 2,412	$ 1,805
Number of Securities available for sale greater than one year \| security	1	3
Securities available-for-sale gross unrealized losses greater than one year	$ 0	$ (3)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 192	$ 898
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities available for sale in one year or less \| security	1	1
Securities available-for-sale gross unrealized losses one year or less	$ (1)	$ (1)
Securities available-for-sale aggregate fair value of investment one year or less	$ 158	$ 158
Number of Securities available for sale greater than one year \| security	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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