Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		$ 31,379	$ 30,700
Available-for-sale securities, Unrealized Losses		(758)	(544)
Available-for-sale Securities, Unrealized Gains		177	263
Available-for-sale Securities, Amortized Cost		31,960	30,981
Securities held-to-maturity, fair value		14,443	13,902
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		(299)	(149)
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		72	74
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		14,670	13,977
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		13,290	15,164
Available-for-sale securities, Unrealized Losses		(365)	(329)
Available-for-sale Securities, Unrealized Gains		106	153
Available-for-sale Securities, Amortized Cost		13,549	15,340
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		5,393	3,539
Available-for-sale securities, Unrealized Losses		(154)	(77)
Available-for-sale Securities, Unrealized Gains		6	0
Available-for-sale Securities, Amortized Cost		5,541	3,616
Securities held-to-maturity, fair value		1,779	2,111
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		(34)	(12)
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		1	10
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		1,812	2,113
Collateralized Mortgage Obligations, Issued by U.S. Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		652	624
Available-for-sale securities, Unrealized Losses		(35)	(24)
Available-for-sale Securities, Unrealized Gains		1	0
Available-for-sale Securities, Amortized Cost		686	648
Securities held-to-maturity, fair value		1,886	1,304
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		(26)	(18)
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		34	41
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		1,878	1,281
Direct Agency Obligations U S Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		2,866	3,454
Available-for-sale securities, Unrealized Losses		(2)	0
Available-for-sale Securities, Unrealized Gains		37	85
Available-for-sale Securities, Amortized Cost		2,831	3,369
Mortgage-backed Securities, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		4,272	5,066
Available-for-sale securities, Unrealized Losses		(171)	(87)
Available-for-sale Securities, Unrealized Gains		1	1
Available-for-sale Securities, Amortized Cost		4,442	5,152
Securities held-to-maturity, fair value		2,043	2,476
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		(63)	(13)
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		2	4
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		2,104	2,485
Collateralized Mortgage Obligations, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		1,120	775
Available-for-sale securities, Unrealized Losses		(29)	(18)
Available-for-sale Securities, Unrealized Gains		9	1
Available-for-sale Securities, Amortized Cost		1,140	792
Securities held-to-maturity, fair value		8,722	7,995
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		(176)	(106)
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		35	18
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		8,863	8,083
Direct Agency Obligations, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		304	438
Available-for-sale securities, Unrealized Losses		0	0
Available-for-sale Securities, Unrealized Gains		9	19
Available-for-sale Securities, Amortized Cost		295	419
Obligations of U.S. states and political subdivision [Member]
Debt Securities, Available-for-sale [Line Items]
Securities held-to-maturity, fair value		11	13
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		0	1
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		11	12
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities held-to-maturity, fair value		2	3
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		0	0
Debt Securities, Held-to-maturity, Amortized Cost, before Other-than-temporary Impairment		2	3
Home Equity Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		42	51
Available-for-sale securities, Unrealized Losses		(2)	(3)
Available-for-sale Securities, Unrealized Gains		0	0
Available-for-sale Securities, Amortized Cost		44	54
Other securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)		107	510
Available-for-sale securities, Unrealized Losses		0	0
Available-for-sale Securities, Unrealized Gains		5	4
Available-for-sale Securities, Amortized Cost		102	506
Foreign Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)	[1]	3,333	902
Available-for-sale securities, Unrealized Losses	[1]	0	0
Available-for-sale Securities, Unrealized Gains	[1]	3	0
Available-for-sale Securities, Amortized Cost	[1]	$ 3,330	902
Equity securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale (includes $822 million and $524 million of assets pledged to creditors at December 31, 2018 and 2017, respectively)			177
Available-for-sale securities, Unrealized Losses			(6)
Available-for-sale Securities, Unrealized Gains			0
Available-for-sale Securities, Amortized Cost			$ 183

[1] Foreign debt securities represent public sector entity, bank or corporate debt.