Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	202	148
Securities available-for-sale gross unrealized losses one year or less	$ (209)	$ (95)
Securities available-for-sale aggregate fair value of investment one year or less	$ 8,002	$ 7,240
Number of Securities available for sale greater than one year \| security	171	154
Securities available-for-sale gross unrealized losses greater than one year	$ (659)	$ (449)
Securities available-for-sale agregate fair value of investment greater than one year	$ 13,251	$ 14,239
Number of securities held-to-maturity in one year or less \| security	522	381
Securities held-to-maturity gross unrealized losses one year or less	$ (191)	$ (53)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 7,764	$ 7,099
Number of securities held-to-maturity greater than one year \| security	753	792
Securities held-to-maturity gross unrealized losses greater than one year	$ (205)	$ (96)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 4,449	$ 3,982
Equity Securities [Member]
Debt Securities, Available-for-sale [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 agregate fair value of investment greater than one year		$ 177
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	13	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	$ 1,141	$ 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 agregate fair value of investment greater than one year	$ 45	$ 44
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [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	$ (2)	$ (3)
Securities available-for-sale agregate fair value of investment greater than one year	$ 47	$ 51
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	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
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	47	52
Securities available-for-sale gross unrealized losses one year or less	$ (146)	$ (85)
Securities available-for-sale aggregate fair value of investment one year or less	$ 4,396	$ 5,161
Number of Securities available for sale greater than one year \| security	40	23
Securities available-for-sale gross unrealized losses greater than one year	$ (72)	$ (20)
Securities available-for-sale agregate fair value of investment greater than one year	$ 1,284	$ 573
Number of securities held-to-maturity in one year or less \| security	184	116
Securities held-to-maturity gross unrealized losses one year or less	$ (153)	$ (48)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 5,976	$ 5,973
Number of securities held-to-maturity greater than one year \| security	475	506
Securities held-to-maturity gross unrealized losses greater than one year	$ (161)	$ (71)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 3,531	$ 2,962
US Government-sponsored Enterprises Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	134	83
Securities available-for-sale gross unrealized losses one year or less	$ (45)	$ (7)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,657	$ 883
Number of Securities available for sale greater than one year \| security	88	77
Securities available-for-sale gross unrealized losses greater than one year	$ (196)	$ (94)
Securities available-for-sale agregate fair value of investment greater than one year	$ 3,016	$ 3,109
Number of securities held-to-maturity in one year or less \| security	338	264
Securities held-to-maturity gross unrealized losses one year or less	$ (38)	$ (5)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,788	$ 1,126
Number of securities held-to-maturity greater than one year \| security	277	284
Securities held-to-maturity gross unrealized losses greater than one year	$ (44)	$ (25)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 918	$ 1,020
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Number of securities available for sale in one year or less \| security	7	3
Securities available-for-sale gross unrealized losses one year or less	$ (18)	$ (3)
Securities available-for-sale aggregate fair value of investment one year or less	$ 808	$ 463
Number of Securities available for sale greater than one year \| security	38	48
Securities available-for-sale gross unrealized losses greater than one year	$ (389)	$ (326)
Securities available-for-sale agregate fair value of investment greater than one year	$ 8,859	$ 10,285

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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