Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) 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	148	421
Securities available-for-sale gross unrealized losses one year or less	$ (95)	$ (604)
Securities available-for-sale aggregate fair value of investment one year or less	$ 7,240	$ 24,619
Number of Securities available for sale greater than one year \| security	154	66
Securities available-for-sale gross unrealized losses greater than one year	$ (449)	$ (217)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 14,239	$ 4,931
Number of securities held-to-maturity in one year or less \| security	381	614
Securities held-to-maturity gross unrealized losses one year or less	$ (53)	$ (86)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 7,099	$ 6,747
Number of securities held-to-maturity greater than one year \| security	792	551
Securities held-to-maturity gross unrealized losses greater than one year	$ (96)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 3,982	$ 133
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	3	40
Securities available-for-sale gross unrealized losses one year or less	$ (3)	$ (378)
Securities available-for-sale aggregate fair value of investment one year or less	$ 463	$ 13,707
Number of Securities available for sale greater than one year \| security	48	32
Securities available-for-sale gross unrealized losses greater than one year	$ (326)	$ (181)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 10,285	$ 3,908
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	83	316
Securities available-for-sale gross unrealized losses one year or less	$ (7)	$ (155)
Securities available-for-sale aggregate fair value of investment one year or less	$ 883	$ 6,474
Number of Securities available for sale greater than one year \| security	77	18
Securities available-for-sale gross unrealized losses greater than one year	$ (94)	$ (13)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,109	$ 427
Number of securities held-to-maturity in one year or less \| security	264	434
Securities held-to-maturity gross unrealized losses one year or less	$ (5)	$ (21)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,126	$ 2,013
Number of securities held-to-maturity greater than one year \| security	284	45
Securities held-to-maturity gross unrealized losses greater than one year	$ (25)	$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 1,020	$ 21
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	52	57
Securities available-for-sale gross unrealized losses one year or less	$ (85)	$ (66)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,161	$ 3,941
Number of Securities available for sale greater than one year \| security	23	7
Securities available-for-sale gross unrealized losses greater than one year	$ (20)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 573	$ 252
Number of securities held-to-maturity in one year or less \| security	116	179
Securities held-to-maturity gross unrealized losses one year or less	$ (48)	$ (65)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 5,973	$ 4,734
Number of securities held-to-maturity greater than one year \| security	506	503
Securities held-to-maturity gross unrealized losses greater than one year	$ (71)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 2,962	$ 112
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	2	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	$ 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	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 \| security	4	8
Securities available-for-sale gross unrealized losses greater than one year	$ (3)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 51	$ 166
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	10	7
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	$ 733	$ 343
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	$ (1)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 44	$ 178
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	1
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (5)
Securities available-for-sale aggregate fair value of investment one year or less	$ 0	$ 154
Number of Securities available for sale greater than one year \| security	1	0
Securities available-for-sale gross unrealized losses greater than one year	$ (6)	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 177	$ 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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