Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Jun. 30, 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	224	421
Securities available-for-sale gross unrealized losses one year or less	$ (420)	$ (604)
Securities available-for-sale aggregate fair value of investment one year or less	$ 18,483	$ 24,619
Number of Securities available for sale greater than one year \| security	56	66
Securities available-for-sale gross unrealized losses greater than one year	$ (119)	$ (217)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,909	$ 4,931
Number of securities held-to-maturity in one year or less \| security	560	614
Securities held-to-maturity gross unrealized losses one year or less	$ (67)	$ (86)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 6,587	$ 6,747
Number of securities held-to-maturity greater than one year \| security	534	551
Securities held-to-maturity gross unrealized losses greater than one year	$ (2)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 383	$ 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	30	40
Securities available-for-sale gross unrealized losses one year or less	$ (237)	$ (378)
Securities available-for-sale aggregate fair value of investment one year or less	$ 8,869	$ 13,707
Number of Securities available for sale greater than one year \| security	27	32
Securities available-for-sale gross unrealized losses greater than one year	$ (97)	$ (181)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 2,970	$ 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	147	316
Securities available-for-sale gross unrealized losses one year or less	$ (105)	$ (155)
Securities available-for-sale aggregate fair value of investment one year or less	$ 4,231	$ 6,474
Number of Securities available for sale greater than one year \| security	15	18
Securities available-for-sale gross unrealized losses greater than one year	$ (5)	$ (13)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 432	$ 427
Number of securities held-to-maturity in one year or less \| security	416	434
Securities held-to-maturity gross unrealized losses one year or less	$ (16)	$ (21)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,696	$ 2,013
Number of securities held-to-maturity greater than one year \| security	46	45
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	$ 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	42	57
Securities available-for-sale gross unrealized losses one year or less	$ (78)	$ (66)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,192	$ 3,941
Number of Securities available for sale greater than one year \| security	6	7
Securities available-for-sale gross unrealized losses greater than one year	$ (6)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 77	$ 252
Number of securities held-to-maturity in one year or less \| security	144	179
Securities held-to-maturity gross unrealized losses one year or less	$ (51)	$ (65)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 4,891	$ 4,734
Number of securities held-to-maturity greater than one year \| security	485	503
Securities held-to-maturity gross unrealized losses greater than one year	$ (2)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 380	$ 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	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	$ 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	6	8
Securities available-for-sale gross unrealized losses greater than one year	$ (5)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 105	$ 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	5	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	$ 191	$ 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	$ (1)	$ (1)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 171	$ 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	$ (5)	$ 0
Securities available-for-sale aggregate fair value of investment greater than one year	$ 154	$ 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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