Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) 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	175	148
Securities available-for-sale gross unrealized losses one year or less	$ (208)	$ (95)
Securities available-for-sale aggregate fair value of investment one year or less	$ 7,616	$ 7,240
Number of Securities available for sale greater than one year \| security	155	154
Securities available-for-sale gross unrealized losses greater than one year	$ (595)	$ (449)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 13,142	$ 14,239
Number of securities held-to-maturity in one year or less \| security	515	381
Securities held-to-maturity gross unrealized losses one year or less	$ (159)	$ (53)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 7,947	$ 7,099
Number of securities held-to-maturity greater than one year \| security	784	792
Securities held-to-maturity gross unrealized losses greater than one year	$ (164)	$ (96)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 4,244	$ 3,982
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	8	3
Securities available-for-sale gross unrealized losses one year or less	$ (13)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 834	$ 463
Number of Securities available for sale greater than one year \| security	42	48
Securities available-for-sale gross unrealized losses greater than one year	$ (394)	$ (326)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 9,375	$ 10,285
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	102	83
Securities available-for-sale gross unrealized losses one year or less	$ (33)	$ (7)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,161	$ 883
Number of Securities available for sale greater than one year \| security	84	77
Securities available-for-sale gross unrealized losses greater than one year	$ (165)	$ (94)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,005	$ 3,109
Number of securities held-to-maturity in one year or less \| security	352	264
Securities held-to-maturity gross unrealized losses one year or less	$ (31)	$ (5)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,860	$ 1,126
Number of securities held-to-maturity greater than one year \| security	288	284
Securities held-to-maturity gross unrealized losses greater than one year	$ (37)	$ (25)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 963	$ 1,020
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	55	52
Securities available-for-sale gross unrealized losses one year or less	$ (162)	$ (85)
Securities available-for-sale aggregate fair value of investment one year or less	$ 4,994	$ 5,161
Number of Securities available for sale greater than one year \| security	24	23
Securities available-for-sale gross unrealized losses greater than one year	$ (33)	$ (20)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 666	$ 573
Number of securities held-to-maturity in one year or less \| security	162	116
Securities held-to-maturity gross unrealized losses one year or less	$ (128)	$ (48)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 6,087	$ 5,973
Number of securities held-to-maturity greater than one year \| security	495	506
Securities held-to-maturity gross unrealized losses greater than one year	$ (127)	$ (71)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 3,281	$ 2,962
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]
Number of securities held-to-maturity in one year or less \| security	1	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	1	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	$ 0
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	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 \| security	4	4
Securities available-for-sale gross unrealized losses greater than one year	$ (3)	$ (3)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 49	$ 51
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	9	10
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	$ 627	$ 733
Number of Securities available for sale greater than one year \| security	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	$ 47	$ 44
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		0
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		$ 0
Number of Securities available for sale greater than one year \| security		1
Securities available-for-sale gross unrealized losses greater than one year		$ (6)
Securities available-for-sale aggregate fair value of investment greater than one year		$ 177

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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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