Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Sep. 30, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security	35	287
Securities available-for-sale gross unrealized losses one year or less	$ (33)	$ (403)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,298	$ 22,754
Number of Securities available for sale greater than one year \| security	64	38
Securities available-for-sale gross unrealized losses greater than one year	$ (255)	$ (93)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 4,823	$ 1,516
Number of securities held-to-maturity in one year or less \| security	128	458
Securities held-to-maturity gross unrealized losses one year or less	$ (2)	$ (36)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,271	$ 4,446
Number of securities held-to-maturity greater than one year \| security	561	709
Securities held-to-maturity gross unrealized losses greater than one year	$ (1)	$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 92	$ 21
U.S. Treasury [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security	15	52
Securities available-for-sale gross unrealized losses one year or less	$ (28)	$ (227)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,110	$ 11,046
Number of Securities available for sale greater than one year \| security	28	5
Securities available-for-sale gross unrealized losses greater than one year	$ (192)	$ (43)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,573	$ 924
US Government sponsored enterprises [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security	13	164
Securities available-for-sale gross unrealized losses one year or less	$ (4)	$ (30)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,683	$ 1,451
Number of Securities available for sale greater than one year \| security	22	19
Securities available-for-sale gross unrealized losses greater than one year	$ (24)	$ (13)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 818	$ 282
Number of securities held-to-maturity in one year or less \| security	14	312
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ (14)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 20	$ 1,143
Number of securities held-to-maturity greater than one year \| security	46	49
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	$ 21	$ 0
US Government agency issued or guaranteed [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security	4	62
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (141)
Securities available-for-sale aggregate fair value of investment one year or less	$ 316	$ 9,725
Number of Securities available for sale greater than one year \| security	5	3
Securities available-for-sale gross unrealized losses greater than one year	$ (6)	$ (8)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 91	$ 101
Number of securities held-to-maturity in one year or less \| security	114	145
Securities held-to-maturity gross unrealized losses one year or less	$ (2)	$ (22)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,251	$ 3,303
Number of securities held-to-maturity greater than one year \| security	512	657
Securities held-to-maturity gross unrealized losses greater than one year	$ (1)	$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 71	$ 20
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security		4
Securities available-for-sale gross unrealized losses one year or less		$ 0
Securities available-for-sale aggregate fair value of investment one year or less		$ 16
Number of Securities available for sale greater than one year \| security		3
Securities available-for-sale gross unrealized losses greater than one year		$ 0
Securities available-for-sale aggregate fair value of investment greater than one year		$ 45
Number of securities held-to-maturity in one year or less \| security	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	$ 0
Number of 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	$ 0	$ 1
Asset backed securities [Member]
Schedule of Available-for-sale and Held-to-Maturity 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	$ 0	$ 0
Securities available-for-sale aggregate fair value of investment one year or less	$ 4	$ 9
Number of Securities available for sale greater than one year \| security	8	8
Securities available-for-sale gross unrealized losses greater than one year	$ (32)	$ (29)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 159	$ 164
Foreign debt securities [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities available for sale in one year or less \| security	1	3
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (2)
Securities available-for-sale aggregate fair value of investment one year or less	$ 27	$ 351
Number of Securities available for sale greater than one year \| security	1	0
Securities available-for-sale gross unrealized losses greater than one year	$ (1)	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 182	$ 0
Equity securities [Member]
Schedule of Available-for-sale and Held-to-Maturity 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)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 158	$ 156
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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-Publisher FASB
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