Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	38	88
Securities available-for-sale gross unrealized losses one year or less	$ (37)	$ (20)
Securities available-for-sale aggregate fair value of investment one year or less	$ 5,521	$ 3,381
Number of Securities available for sale greater than one year \| security	218	276
Securities available-for-sale gross unrealized losses greater than one year	$ (331)	$ (738)
Securities available-for-sale agregate fair value of investment greater than one year	$ 12,837	$ 17,393
Number of securities held-to-maturity in one year or less \| security	44	215
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ (22)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 190	$ 1,453
Number of securities held-to-maturity greater than one year \| security	688	1,003
Securities held-to-maturity gross unrealized losses greater than one year	$ (51)	$ (277)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 5,979	$ 9,111
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	17	8
Securities available-for-sale gross unrealized losses one year or less	$ (31)	$ (12)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,652	$ 1,013
Number of Securities available for sale greater than one year \| security	32	41
Securities available-for-sale gross unrealized losses greater than one year	$ (233)	$ (353)
Securities available-for-sale agregate fair value of investment greater than one year	$ 6,030	$ 8,828
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	1	55
Securities available-for-sale gross unrealized losses one year or less	$ 0	$ (6)
Securities available-for-sale aggregate fair value of investment one year or less	$ 252	$ 538
Number of Securities available for sale greater than one year \| security	122	159
Securities available-for-sale gross unrealized losses greater than one year	$ (32)	$ (185)
Securities available-for-sale agregate fair value of investment greater than one year	$ 2,678	$ 3,698
Number of securities held-to-maturity in one year or less \| security	12	103
Securities held-to-maturity gross unrealized losses one year or less	$ 0	$ (8)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 3	$ 684
Number of securities held-to-maturity greater than one year \| security	301	511
Securities held-to-maturity gross unrealized losses greater than one year	$ (10)	$ (52)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 1,210	$ 1,739
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	2	8
Securities available-for-sale gross unrealized losses one year or less	$ (6)	$ (2)
Securities available-for-sale aggregate fair value of investment one year or less	$ 177	$ 106
Number of Securities available for sale greater than one year \| security	59	72
Securities available-for-sale gross unrealized losses greater than one year	$ (63)	$ (198)
Securities available-for-sale agregate fair value of investment greater than one year	$ 4,093	$ 4,825
Number of securities held-to-maturity in one year or less \| security	32	111
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	$ 187	$ 769
Number of securities held-to-maturity greater than one year \| security	386	491
Securities held-to-maturity gross unrealized losses greater than one year	$ (41)	$ (225)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 4,769	$ 7,372
Asset-backed securities [Member]
Debt Securities, Available-for-sale [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	$ 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	5	4
Securities available-for-sale gross unrealized losses greater than one year	$ (3)	$ (2)
Securities available-for-sale agregate fair value of investment greater than one year	$ 36	$ 42
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	18	16
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	$ 1,440	$ 1,724
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 agregate fair value of investment greater than one year	$ 0	$ 0
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [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	1	1
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

Held to maturity securities continuous unrealized loss position less than twelve months number of securities.

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Held to maturity securities continuous unrealized loss position twelve months or longer number of securities.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

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