Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Mar. 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	501	421
Securities available-for-sale gross unrealized losses one year or less	$ (546)	$ (604)
Securities available-for-sale aggregate fair value of investment one year or less	$ 20,479	$ 24,619
Number of Securities available for sale greater than one year \| security	59	66
Securities available-for-sale gross unrealized losses greater than one year	$ (143)	$ (217)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 3,885	$ 4,931
Number of securities held-to-maturity in one year or less \| security	632	614
Securities held-to-maturity gross unrealized losses one year or less	$ (89)	$ (86)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 6,851	$ 6,747
Number of securities held-to-maturity greater than one year \| security	537	551
Securities held-to-maturity gross unrealized losses greater than one year	$ (1)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 82	$ 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	36	40
Securities available-for-sale gross unrealized losses one year or less	$ (318)	$ (378)
Securities available-for-sale aggregate fair value of investment one year or less	$ 10,241	$ 13,707
Number of Securities available for sale greater than one year \| security	28	32
Securities available-for-sale gross unrealized losses greater than one year	$ (117)	$ (181)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 2,867	$ 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	408	316
Securities available-for-sale gross unrealized losses one year or less	$ (153)	$ (155)
Securities available-for-sale aggregate fair value of investment one year or less	$ 6,468	$ 6,474
Number of Securities available for sale greater than one year \| security	17	18
Securities available-for-sale gross unrealized losses greater than one year	$ (7)	$ (13)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 431	$ 427
Number of securities held-to-maturity in one year or less \| security	449	434
Securities held-to-maturity gross unrealized losses one year or less	$ (22)	$ (21)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 2,053	$ 2,013
Number of securities held-to-maturity greater than one year \| security	43	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	$ 21	$ 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	55	57
Securities available-for-sale gross unrealized losses one year or less	$ (75)	$ (66)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,663	$ 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	$ (7)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 151	$ 252
Number of securities held-to-maturity in one year or less \| security	182	179
Securities held-to-maturity gross unrealized losses one year or less	$ (67)	$ (65)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 4,798	$ 4,734
Number of securities held-to-maturity greater than one year \| security	491	503
Securities held-to-maturity gross unrealized losses greater than one year	$ (1)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 61	$ 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	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	$ (7)	$ (11)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 107	$ 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	2	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	$ 107	$ 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	$ 175	$ 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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