Securities - Gross unrealized losses and related fair values (Detail) $ in Millions	Sep. 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	162	421
Securities available-for-sale gross unrealized losses one year or less	$ (364)	$ (604)
Securities available-for-sale aggregate fair value of investment one year or less	$ 13,254	$ 24,619
Number of Securities available for sale greater than one year \| security	57	66
Securities available-for-sale gross unrealized losses greater than one year	$ (141)	$ (217)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 4,383	$ 4,931
Number of securities held-to-maturity in one year or less \| security	378	614
Securities held-to-maturity gross unrealized losses one year or less	$ (56)	$ (86)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 6,208	$ 6,747
Number of securities held-to-maturity greater than one year \| security	520	551
Securities held-to-maturity gross unrealized losses greater than one year	$ (13)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 792	$ 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	28	40
Securities available-for-sale gross unrealized losses one year or less	$ (214)	$ (378)
Securities available-for-sale aggregate fair value of investment one year or less	$ 7,433	$ 13,707
Number of Securities available for sale greater than one year \| security	29	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	$ 3,675	$ 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	84	316
Securities available-for-sale gross unrealized losses one year or less	$ (80)	$ (155)
Securities available-for-sale aggregate fair value of investment one year or less	$ 3,291	$ 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	$ (9)	$ (13)
Securities available-for-sale aggregate fair value of investment greater than one year	$ 259	$ 427
Number of securities held-to-maturity in one year or less \| security	253	434
Securities held-to-maturity gross unrealized losses one year or less	$ (13)	$ (21)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 1,156	$ 2,013
Number of securities held-to-maturity greater than one year \| security	51	45
Securities held-to-maturity gross unrealized losses greater than one year	$ (1)	$ 0
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 16	$ 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	$ (70)	$ (66)
Securities available-for-sale aggregate fair value of investment one year or less	$ 1,858	$ 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	$ 73	$ 252
Number of securities held-to-maturity in one year or less \| security	125	179
Securities held-to-maturity gross unrealized losses one year or less	$ (43)	$ (65)
Securities held-to-maturity aggregate fair value of investment one year or less	$ 5,052	$ 4,734
Number of securities held-to-maturity greater than one year \| security	467	503
Securities held-to-maturity gross unrealized losses greater than one year	$ (12)	$ (1)
Securities held-to-maturity aggregate fair value of investment greater than one year	$ 776	$ 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	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	5	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	$ 54	$ 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	8	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	$ 672	$ 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	$ 168	$ 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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